It seems to me that the rate of evaporation during the boil is a function of energy input and surface area of the wort. It's not proportional to the initial volume. So the boil-off calculation really should be expressed as gallons/hour rather than % of initial volume.

To get around this problem, I've had to create separate equipment entries for different boil times. If I use the same equipment profile for 60 minute and 90 minute boils, the result is that my beers boiled for 90 minutes end up over volume.

Any comments?

Thanks.