johnnymac wrote:In my second example you'd have a higher preboil number on the larger batch so I guess it would work out to be the same.
You got it. But we really only care about the pre-boil number in terms of using it to predict the post-boil number (okay, it does affect hop utilization too, but the software takes care of that for you).
I find it helps to figure out what you want to end up with and work backwards from there--on my system, I know I need 6.3 gallons in the kettle post boil, so I add the boil-off amount to that (say, 1.5 gallons). So I know I must have 7.8 gallons pre-boil. Now I compute that to end up with 6.3 gallons of 1.050 wort requires a pre-boil gravity of 1.059, and can get the appropriate grain amount based on my efficiency to hit this.
Hope that makes sense, I'm happy to clarify if not.