Originally Posted by Luke_M
I just built a mash tun using a Rubbermaid cooler and will be batch sparging
Sounds like my set up. Here's my suggestion - I am sure there are better ways, but most of my brew days are a variation on this:
Heat around 5-6 gallons of strike water to 170 deg F. At 170, put dough-in water in your cooler. Put in 1 quart for each pound of grain. Don't put the grain in yet... Let the hot water pre-heat your cooler for you. When water reaches 155-160 deg F, put in all of your grain. You should end up with mash around 130-135. Let this rest until the balance of your strike water is boiling.
Once strike water is boiling, add enough to get your mash to desired saccharification temp. Be careful to not overshoot and be patient. You should end up with liquor to grist ratio of 1.5 to 2.0. Don't worry about ratio - worry about the temp if anything. If its too cold, add more boiling water. If too hot, leave the lid of you cooler open and stir gently.
After sacc rest is done (60 min? Use iodine test if you aren't sure about this), you add the make-up water. I start heating it so that make up water is boiling right about when sacc rest is complete - you'll want about 4 gallons ready to use if necessary. Now the tricky part - you want to add enough water to get the gravity of the sweet wort to your desired pre-boil gravity.
Your wort will concentrate during the boil (as water evaporates away). If you have a feel for what that might be, you can calculate this a bit more exactly. I would suggest using something like 15% as a starting point. You want your pre-boil gravity to be around 15% lower than your target gravity. So... Now you start this cycle of adding boiling water, stirring gently, and check gravity until you hit your desired pre-boil gravity. A refractometer is a very handy tool to have at this stage.
You don't want to get your mash over 170 here, but my experience is that it's just about impossible to make that happen unless your making a lower gravity beer (like <1.040).
All done! Let your mash rest a bit then vorlauf for 10+ minutes before you start lautering into your kettle. Now use the length of your boil to hit the target gravity. Biggest risk is that you don't use enough make up water and you hit your target gravity too early in the boil. If that happens add more water directly to the boil. No problem!
You might find that your ending volume is a little low. I would guess that will be the case because of you are using a kit and your (our!) method is not the most efficient. I usually spend a few extra $'s on base malt to scale up the grist a little bit to make sure I get enough sugars into the wort to get both a full volume and the target gravity. No sweat, though - I would not add water to hit the volume you want and end up with a lower gravity. I usually have some light dry malt on hand in case I really mess it up and under-shoot the gravity substantially, but this should not be that case if you follow the method above.
There's probably plenty of folks smarter than me who will point out the flaws in my method. Maybe you and I will both get some good pointers!