I've got a 10 gallon pot that loses about 2.25 gal per hour to evaporation. Preboil is about 7.5 gal (which I measure with an oak dowel that I marked with Sharpie..works every time)....I use Fermcap-S to control the hot break and foaming. I also back the flame off so it doesn't foam as much while it's boiling. I'd love to have a 15-20 gallon Blichmann kettle and burner, but I make great beer with what I've got. We as brewers have to adapt to our equipment, somewhat. BTW, I think you're going to find that evaporation rates of well over 20% are fairly common.