Turns out this was a combination of using the same float value constants & operation order.
I've reverse engineered the calculation and PKHeX now emulates the entire call chain correctly, as implemented in this commit:
https://github.com/kwsch/PKHeX/commit/9c5955c10404caa9170b7423e39a773212765e8a
This issue is now resolved; un-sticky'ing. Have fun with all the h/w calc precision