Your first thought is correct, you calculate your water volumes so that between mash and sparges you have a specific pre-boil volume. For most typical grain bills I calculate for 6.75 - 7 gallons so that with boil off and loss to hop absorption (mainly) and any trub (depending on the extra ingredients of certain recipes) I get 5G into the fermenter. For bigger beers I find I have to use a *little* more sparge water to get a really good rinse of the grains. Any excess wort I just boil off before adding my first hop addition. Generally I would avoid adding water in most cases. If you come up a little high just deal with it and adjust for it next time around. If for some reason you messed up and came out way high in OG you can boil some water (calculated of course) , cool it, and add it to the wort before you aerate and add yeast. But overall you really shouldn't need to do that if you've calculated everything and come close to your target. Basically I've never needed to do it since I went AG/full boils (ie. off of stove top) from the start. The calculators out there are so incredibly accurate.
Rev.
