Hop usage gets more efficient as volume increases, right? Why is that so and what is the formula to calculate this increased efficiency? (e.g., in Beersmith if 10 liters of beer get 25 IBU, 100 liters of the same beer gets 26 IBU).
I think you missed the placement of the decimal point. I changed only the volume of several recipes just to see what happened and they all came out as they should have. 10 X the volume = close to .10 times the IBUs, with the differences as expected due to gravity. As an example, 6 gallons w/38.7 IBUs and increasing volume to 60 gallons netted 4.2 IBUs. This assume you are changing only the volume too. If not please re-post and be more specific.
No error here. Obviously gravity needs to remain constant. But efficiency increase seems to be only log10(volume)in points ibu