To figure this out I would start from what I want to end up with, then add all losses to determine what I need to start with.
For example, to get 5 gal in the keg, I’d plan 5.5 gal in the FV for trüb loss, 6.5-7 gal post mash to allow for boil off and trüb loss, starting with 8.5 gal to allow for grain absorption. Of course, this isn’t actual data. You’ll need to use your actual data to get your results.
I am sure there are BIAB calculators out there to be more precise.
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