Is it generally as simple as taking the original, example 5 gallon batch, and dividing the individual ingredients by 5, then multiply by whatever new batch volume desired to get the “new” grain/hop bill?
I have ingredients from a Barleywine kit, that I want to par down from 5 gallons to 3 gallons. Then use part of the leftover grain to make a light session brew. I was going to partigyle that full grainbill on the barleywine but have since decided I’d rather ammend the leftover grain for a different style altogether for the second brew.
If you’re doing all-grain you might want to run everything through a calculator just to be sure. Your boiloff rate is going to about the same regardless of batch size, so your mash and/or sparge volumes can’t simply be changed to 60% of your original volumes.
Also, your efficiency may be a bit higher since you will either need a thinner mash or more sparge water relative to the amount of grain used. Hop utilization may increase a bit as well since the boil will start out less concentrated. I don’t think either of these effects will be significant enough to have much of an effect on the end product, but they are something to keep in mind.
