I agree, it doesn’t make sense to me to add salts based on pre boil volume, volume in fermenter, or volume in package, either.
I usually figure the total salts for the profile I am trying to achieve based on post boil volume in the kettle.
I then designate all salts for the boil kettle except the qty of CaCl or gypsum (or a combination) for the Calcium I need to hit 5.2 pH in the mash tun and help the enzymes (~1 tsp).
FWIW, for a typical 5 gal batch packaged, I start with 9 gal, mash with 5 gal, batch sparge with 4 gal, resulting in 7.5 gal pre boil for 6.5 gal post boil (where I target water profile). I transfer 5.5 gal to the fermenter and package 5 gal. Again, these are generalizations, but in the neighborhood.
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