Why is boil off specified as a percentage?

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.

Couldn't agree more Elipsis, hopefully Brad will be implementing g/hour in the next upgrade.

Cheers
Andrew

I just want to add my vote for this.

I switch back and forth between 5 and 10 gallon batches on the same equipment lately, which magnifies the issue.  I can set up separate equipment profiles with dramatically different boil-off rates, but that's a band-aid fix (and murphy's law says I'll forget and select the wrong one all the time)