Software ~ I use Beersmith; and needed a few batches to adjust the parameters to my "system" and method (squeeze bag, drip, beat with stick, fine grind, coarse grind, ? ? ); and experimented with my personal kettle boil off as well. Unfortunately, you can use software all over the web, but you it just an estimate.. you need to Brew, take good notes, and adjust to your method. After a few batches; I am spot on!
Two gallon batch ?
~ BIAB : All Grain Made Easy ; Mash, Sparge, Boil all in the same Kettle ~ all you need is a bag and a hook!