Actually, you CAN enter 0.02 for the Iron entry - the cell is set to display only one decimal point, but if you enter 0.02, click another cell and then re-select the Iron entry, you'll see up top that it's still set to 0.02 and thus wil enter the calculations properly. This will have an effect on your cation/anion values. As for the Flouride and Nitrite entries, I also have them blank...although if your cation/anion ratio is off, you might want to try entering small values (particularly the Fluoride if you're using tap water) to balance your ratios. It's important to start with a balanced ratio for the spreadsheet to work right.
For accurate synthesis it is essential that the source profile be balanced - you can't match an unbalanced profile well. But well balanced source profiles are unusual because of the cumulative effects of errors in the measurements process (and Ward Labs miscalculates carbonate and thus balance). If the report does not balance you cannot calculate a balanced synthesis from it as anything and everything you add is balanced. Guessing whether the imbalance is due to neglect of an ion or measurement or calculation and fudging in something based on a guess to force balance will not get you a better syntheses unless you are incredibly lucky. It might get you a worse one.
Brewing knowledge and literature have come a LONG way in the last decade or two - many old "rules of thumb" were assumptions based on observations or were "borrowed" from similar industries. The 5.2 pH is one of these. In the earlier days of homebrewing, folks discovered that the home winemaking industry had a wonderful product to stabilize things to a nice pH of 5.2 that didn't impart off flavors or otherwise mess with the chemistry - and 5.2 Stabilizer entered the homebrewing market,
I won't say "No" to that but I'm skeptical. Given that 5.2 is sold in homebrewing shops most of which also sell to wine makers I'd expect the packaging to say something about its use in wine making if that were part of its provenance. Besides which it appears, from it's formulation, that it depends on malt phosphate as part of its buffer system. This conclusion is strengthened as it doesn't buffer to anywhere near 5.2 in distilled water. Wine must does not (AFAIK) contain phosphate at anywhere near the levels of a cereal mash. More important than where it came from is that it is ineffective at bringing pH to 5.2 in mash even at 3 and 4 times the recommended dose. And it does add a lot of sodium - at the dose it would take to get close to 5.2 with water of nominal alkalinity that sodium would be tasteable -especially if the water contained any chloride to speak of.
followed by recommendations in publications/books, which led to a flat assumption of 5.2 being the "ideal" pH for a mash and continuing it through the boil.
You may have seen 5.2 stated as the ideal in some home brewing literature but it has been recognized for some time (as early as pH measurements were being made in the brewery) that there is no optimum pH as each of the several enzymes involved in mashing has its own ideal pH and temperature range. I often mention in this context the XII DeClerck Chair which was title "The pH Paradox" with the paradox being that what is good pH for hops utilization isn't good for colloidal stability or mash efficiency...
Most mashes do benefit from having a mash a little higher in pH, then having the pH dropped a bit before the boil. You can choose to do it either way.
There is a lot of confusion abroad relating to the temperature dependence of mash pH. Ales seem to come out best if mashed at pH 5.3-5.5 and lagers at perhaps 5.4 - 5.6 both as measured at room temperature. A typical lager mash will have a pH that decreases 0.0055 pH/°C so that if at strike the pH was 5.6 at the beta glucan rest it might be 5.5, at the protein rest 5.4 and at the saccharification rest 5.34. For an ale it might be as low as 5.2 or even lower at saccharification rest temp.
You do not really have the option of lowering mash pH in the mash tun or the kettle. pH needs to be properly set in the mash tun. That's where the enzymes do their jobs. Kettle pH, or rather pH going into the fermenter is important too and some brewers will add additional acid or salts to the kettle to get the pH down to 5 - 5.2 (measured at room temperature) going into the fermenter.
Personally, I don't keep acids on hand, so I target my mash to be around 5.1-5.4 and don't acidify my strike/sparge water (yes, Bru'un Water lumps them together) - it's much simpler and my beers still come out great.
Good chance they would be better if you controlled mash pH and it takes acid to do that for most beers.
OP does not need to treat sparge water separately as his alkalinity is low enough that he is unlikely to break pH 6 before sparging down to desired minimum runoff extract.
On the Water Adjustment page, yeah....it's like every other one that requires some trial and error. If you talk to a pro, they'll tell you that the only true way to match a water profile exactly is to go get that water - even if your numbers match, it won't be identical (how does one replicate the effect of water slowly filtering through limestone over the years by just dumping in some powders?).
You can't do it with just powders because you cannot buy calcium bicarbonate powder and calcium bicarbonate is a large part of most brewing waters. You can do it with calcium carbonate and CO2 gas - that is how nature did it. Give me a real (balanced) target profile and I can usually match it to within 1% or less error in all ions using chalk, CO2 and the common salts (calcium carbonate, calcium chloride, calcium sulfate, sodium chloride, sodium bicarbonate etc). It's a lot of trouble and seldom worth the effort as you don't need to duplicate Munich water to brew good Helles. In fact you can probably make a better Helles with a water profile different from that of Munich's (but not too much different or it woudn't be a Helles).
I don't keep Magnesium Chloride or Pickling Lime on hand, so I just ignore those. I start with adjusting the Epsom Salt, as that's the only option left to adjust Mg.
One of the ways to improve on a natural profile is to eliminate the magnesium. It is not flavor positive stuff. There is plenty in the malt for enzyme co-factor duty.
Next comes the Gypsum as the only other option to up the Sulfate. Those are the easy ones. My process next goes to Calcium Chloride for the Chloride adjustment, then adding Chalk until my Calcium is right. I add Baking Soda until my Sodium looks good.
One should never add chalk nor sodium bicarbonate unless he is willing to do the CO2 bit I referred to above as without it the chalk will not dissolve but its alkalinity will still be there to screw up your mash pH (the malt acids will go to dissolving chalk and be unavailable to establish proper mash pH) and the calcium will not be in solution to offset it. As most people don't want to do the CO2 and as it is seldom justified any way the practical rule is "Never add carbonate or bicarbonate to mash or mash water unless a mash pH reading made with a recently calibrated meter indicates that mash pH is too low".
At that point, I'm usually pretty darn close to my target profile ....
You may think you are but you aren't. The undissolved calcium is not available. And you cannot match a natural profile without CO2. IOW you cannot match what nature has done unless you do what she did.
The spreadsheets are fine for many purposes but they do have limitations. They are fine on calculating how much calcium and how much chloride is added to a solution by adding a given amount of calcium chloride. When carbonate or bicarbonate come into the picture it becomes an order of magnitude more complex because the pH of the source and of the synthesis become major factors and the problems require iterative solution. That is possible as Excel has its Solver which will do the iterative solution for you but you don't want to mess with that. Even if you did the solutions only describe what happens at thermodynamic equilibrium and systems involving carbonic/bicarbonate/carbonate can take a long time to come to thermodynamic equilibrium. A glass of beer that still has some fizz an hour after it is poured is an example.
