When I punch my water profile into Palmer's nomograph based on 100% base malt mash I get approximately 12 SRM...
Forget about this. I have brewed beers which range in color from 4.3 SRM (the lightest I was ever able to get) and 84 with water which is very similar. Water chemistry has little (i.e. it is only loosely correlated) effect on beer color.
...at a pH between 5.7 and 5.8.
The water doesn't have enough mineral content to shift the pH much relative to a distilled water mash - perhaps 0.08 pH. If your base malt were typical Pilsner malt the distilled water pH might be about 5.75. I have recently measured the DI pH of Maris Otter at 5.60. Your water would take these up to, respectively, 5.83 and 5.68. This, of course, represents a rough estimate. It depends on the malt. You could brew at these levels but would in all liklihood get a better beer if you lowered it some.
Based on what I've read pH will drop approx .35 when at mash temperatures which would put me between 5.35 and 5.45, which would be great, yes?
The drop depends on water and malt and what you consider mash temperature. Calling 21 °C room temperature and 65 °C mash temperature you might expect the drop to be between 0.18 to 0.20.
maybe even a little lower to get within 5.2-5.4?
There is plenty of debate about what the proper pH range is and part of that is stimulated by the fact that authors (with some exceptions) have not been consistent in stating whether pH numbers are for room or mash temperature and if at mash temperature, which mash temperature. This situation has improved recently. You should shoot for a mash pH in the 5.4 - 5.5 region at room temperature (which, of you want to convert to 65 °C would probably get you 5.2 - 5.3 but as you'll never know unless you are willing to sacrifice a pH electrode and measure at that temperature just shoot for that range at room temperature).
It is a good idea to lower your mash pH some. I'm guessing that you will be using a pale ale malt for base and as such expecting a DI water mash pH of around 5.65 - 5.7 (i.e. not as low as Maris Otter - if you are using that then expect 5.6). To get to 5.4 (measured at room temperature) the simplest action is to add sauermalz at the rate of 1% of the grist for each 0.1 pH drop you want to achieve which amounts to 2%-3% in this case. This rule of thumb is a rule of thumb and thus not guaranteed to get you spot on. I never miss an opportunity to advise the purchase of an inexpensive pH meter and its use in checking DI water pH and the effect of sauermalz additions but I understand that you are starting out here and may not want to add another complication to your brew day.
I'll be using flaked corn/rice in my next batch (cream ale) how does one account for these adjuncts and their affect on the mash pH?
The only way to account for them with certainty is to make a test mash without these adjuncts (add-junks as a fellow Reinheitsgebot adherent used to call them), measure its pH and repeat for another test mash with. As they are not roasted I would expect them to behave as do the lightest base malts i.e. I wouldn't expect much of a shift from them with a Pils base but with a pale ale malt base the pH might go up a bit. This is pure speculation on my part. You have probably guessed from the wisecrack that I don't use them.
This cream ale will potentially be a beer I enter in contests so I'd like to try to brew to style, that being said the BJCP guidelines call for an SRM between 2.5 and 5, a bit lighter than the styles the nomograph says are ideal with my water.
This is definitely not a problem. You will never hit SRM 2.5 with home brewing equipment (Bud is 2.4, Miller light 2.8, St Pauli Girl 3.2) but it won't be because of the water. Water chemistry doesn't change beer color (except perhaps for a red shift if there is lots of bicarbonate). What it can do is cause the brewery to add dark malts for their acid hence the correlation, weak though it may be, between color and water composition. You will need to lower pH, as discussed earlier but you won't be doing it with dark malt.
If I'm understanding even a little bit, it would be beneficial for me to bring down the mash pH just a bit further and I've got a couple options to do that, gypsum, calcium chloride, epsom salt or acid malt. Looking at the nomograph adding gypsum or epsom salt raises my sulfate levels pretty significantly, probably moreso than I would like in a cream ale, yes?
Yes, and that's why I recommend sauermalz (acid malt) though I am not familiar with the style. The name suggests that soft water might be desirable. Just as a general comment: addition of calcium for control of pH is of limited effectiveness because it takes so much. And when all the malt's phosphate has been exhausted you can add all the calcium chloride you want and the pH won't drop any further.
Based on my grist, EZ water calc shows that if I were to add 1 gram of calcium chloride my chloride to sulfate ratio would be 1.67 and put me into the malty category but estimates my mash ph as 5.57 (Does this calculator assume mash or room temperature?)
Room temperature AFAIK. Chloride to sulfate ratio is another element of this subject that is overblown. Learn to recognize the effects of chloride and sulfate separately. They are not antipodal as the CSR ratio adherents would have you believe.
Based on the nomograph, adding the above mentioned salts also looks to be making the water more desirable for lighter beers.
This is a perfect example of how these spreadsheets can lead one astray. It would make your water less desirable for lighter beers because they are best when made with soft water (unless you are shooting for something like an Export). The spreadsheets assume that because the water of Pilsen (very soft) would not lower mash pH into the proper range the Pilsen brewers must have added minerals to get the pH into the right range even though the authors know perfectly well that they didn't - they used soured mash (i.e. sauermalz) and that's how the breweries (that still follow Reinheits gebot) do it today. pH's with minerals. This isn't because the spreadsheet makers are lazy or stupid. It's because they need a simple model which applies to all beers. In brewing one size does not fit all any more than it does in any other field.
That being said I think the bigger issue here is that the tap water is not very desirable to use for the light cream ale (Is it safe to say because the water is too hard?). I imagine this is where diluting with RO water or starting with RO water comes into play. How hard should I try to adjust my tap water before I throw the hands up and build up RO water or dilute with RO water?
I think this is very perceptive of you. Light colored, delicate beers shouldn't taste of minerals and so seem to come out better when made with soft water but mash pH control is as or more important. As I mentioned I have never brewed a cream ale but when I brew Pilsner, Kölsch or wheat beer I generally take 10% well water (very similar to yours) and blend it with RO water and supplement the calcium to about 30 mg/L using calcium chloride.
Were I you contemplating brewing this beer I would probably do exactly that which is in essence what the Primer directions for soft water beers advises but that's my take on it which isn't everyone's but I am getting good reports back from people using the Primer.
There is a real danger that beginners will, correctly perceiving that water is a very important part of beer, incorrectly think they must precisely adjust it and consequently over engineer their water treatment. Most of the guys that promulgate these spreadsheets are engineers and I am one too. I also have a spreadsheet and I use it all the time but I don't advertise it because I fear it would do you more harm than good (though it doesn't contain any mash pH or color vs RA models - IMO those things can't be modeled). Basically I brew by the Primer and I like the beers which isn't to say that you would but I think there's a pretty good chance you would.
