Up until their recent upgrade Hopville's Beer Calculus has never steered me wrong. Unfortunately, now there is no possible way to input a boil size that is different from the batch size. This really sucks, since I do 7 gallon boils to make 15 gallon batches. IE I need to put in considerably larger hop additions in a boil slightly more than half the volume of the diluted final batch. Understandably, isomerization is markedly different at varying specific gravities, and calculating the size of my hop additions in an extremely concentrated high gravity boil to make X amount of IBU's in a finished product post dilution is not as simple as plain multiplication.
Anyone know of anything that can do this math for me? For free?
If anyone has an actual formula, that would likely be just as helpful.