Originally Posted by cokronk
How do I figure this out?
You can certainly figure it out w/ software or by hand if you know the maths and have the desire...A very simple method is just to measure your wort in the ketttle before the last sparge and subtract that from your preboil volume giving you the amount of wort needed to reach your preboil volume, this will be the same volume as needed for your last sparge...easy peasy.
So to put it into rough easy numbers, say for a 10 gallon batch you want 13 gal preboil, figuring on boiling off 2 gallons and losing a gallon to trub, leaving 10 gallons finished beer. If prior to your last sparge you have 9 in the kettle, the last sparge would be 13 minus 9, or 4 gallons.
With experience you will learn losses due to boiloff, trub/hops sludge and yeast cake to better figure for the final batch.