I decided to drop the platoon matchup. In 2022 switch hitters hit .231 as a group (against a dismal-enough MLB average of .242) and it doesn't seem fair to give bad hitters a handicap (or good hitters a penalty). I could adjust the averages up or down so each group collectively hits the MLB average for the sake of platoon environments, but that might open up other cans of worms and seems like more trouble than its worth anyway.
So, here are the formulas:
xBA (expected batting avg.) = batterBA + pitcherBA - mlbBA
HAA (hits above average) = H - xBA
trueH = AB x mlbBA + HAA
trueBA = trueH / AB
When Shohei Ohtani (.304 BA) led off the 8th inning of the Angels' 2023 home opener against Toronto's Erik Swanson (.213 BA allowed), the xBA was .304 + .213 - .248 = .269.
Ohtani doubled, so the at-bat generated 1 - .269 = .731 hits above average.
Angel Stadium had -11 hits above average in 2023, in 5,515 at-bats, so it had 5,515 x .248 - 11 = 1,358 "true" hits.
And therefore the Big A's trueBA was 1,358 / 5,515 = .246.
Dropping the platoon matchup hardly changed the parks' trueBA at all: the highest of the thirty regular parks was still Coors Field, .270, while the lowest was still Citi Field, .235.
In 2022, the highest was Coors Field again, at .261, while the lowest was Petco Park, .229.
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