I know this has to have showed up but a search only throws me people wondering why it's not in (or it being added as a minor point in bigger suggestions).
CON (and/or AGI) should be added to increase total Stamina or decrease Stamina drain, preferably the former.
The fact that stamina is purely limited with no way of increasing it is a pain in the ass. It should definitely give diminishing returns (like the quality formula).
Something simple like 5(Base gain) * sqrt((CON+AGI)/5)
You should need considerable gains in both to get over 200% Stamina (500 in each?)