Based on the airport’s country or if it’s easier, the METAR.
The weights and distances make sense as something not auto-selected, since many people will invariably use imperial airframes in metric countries and vice versa, due to personal preference and familiarity, but for the most part I think it makes sense to match the altimeter unit in use at a given airport (while letting the user override it as desired).
This would happen automatically when entering the airport identifier resp. choosing “populate from last flight”, I think?
Edit: I wanted to post this in Wishlist
Regards,
Tim