hey Ken, you don't need to calculate from the bottom of the joists to the drywall, you calc from the bottom of the floor (the underside of the sub-floor) to the drywall.
if you had a very large area that was within 1" of the ceiling back, that would have to be considered, but a narrow surface area like the bottom of a 2x10 can be neglected.
boy, lemme try to think of an analogy - think of it as just compressing the air in the space between floor and ceiling... pushing the drywall up compresses... the tiny space between the joists and the drywall - the air won't compress in that space, it will just push out into the larger part of the air cavity.
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