This calculation is presented in John Palmer’s How to Brew for scaling up a regular-sized all-grain recipe to the size needed for the batch sparging technique he presents. Normally, recipes assume an extract potential of about 36 ppg for all-grain brewing, but the batch sparge technique instead yields 28 ppg; this calculation will tell you what your scaled-up grain bill for the recipe should be, as well as how much water you’ll need for the mash.