I think you've pretty-much gotthe first question down.
Originally Posted by jayhoz
Is there is a minimum volume (ratio to amount of grain) recommended for the sparge? For one particular recipe with 16lbs of grain I calculated a sparge volume of 3.8 gallons using the logic above. Should I bump this up to a 90 min boil in order to get the sparge volume up to 4.5 gallons?
The answer is yes, but I can't tell you what the ratio is. I think a lot of brewers kindof do this by the seat of their pants. I mostly do moderate gravity brews, and I'm usually shooting for 7.5 gals in the kettle, which ends up being about 5.5 gals in the primary.
If that ratio gets too low, then you're not getting all of the sugars out of the grains. Fly spargers can just keep sparging until a certain SG (or pH) of the runnings is reached. For batch spargers, I think it's more of a seat of the pants thing.
16 lbs is about the biggest batch I've done--I collected about 8 gallons when I did a tripel with that amount of grain.