If I were to take any issue with the "averaging around 20% per fight, and 80% per day", even if the split ends up like...10%, 15%, 25%, and 30%...that has the adventurers resting for the day with about 20% of their resources left. If they get attacked while resting, is it gonna be another 10% they can wipe easy, a 15% that'll put that at risk, a 25% that might well wipe the party, or a 30% that will almost assuredly wipe the party? When they choose to rest is semi-within their control, and in general, I'd probably choose to rest if I fell below...probably about 40%? Maybe a bit lower on spells/consumables, depending on how good the ones I've got left are, but definitely for HP, I'm gonna be kinda nervous going into a fight with less than half health and the cleric going "sorry I'll have more healing tomorrow".