I don't know how anyone ever actually uses the pre, post and fermenter volume calculations in the recipe builder. It's complete nonsense. If I select kettle as target and use a figure that I know to be correct for the post boil and hit calculate, it changes both the pre and post boil numbers. If I use a pre boil volume that I know to be correct it still changes both numbers. In either case neither number stays at the input value and neither is correct based on my system profile. It's idiotic and I can't say often enough how frustrated and disappointed I am at what this recipe builder has become. Some aspects of it are a complete time suck and don't yield any useful information. Totally over it. Fortunately I have my own spreadsheet that calculates my volumes to a tee based on my system (same exact system values that I use, to no avail, in the BF calculator).