I'm gonna just show this suggestion from Ouatcheur that I think would work well:
Quote from Ouatcheur
Personally, I'd make all those boosts lilke this:
Fully drawn bow: 100% of the matgical boosts are added.
Not fully drawn bow, between 50% drawn and 99% drawn: only half of the total strength of the boosts gets added.
Not fully drawn bow, less than 50%: no magical boost.
Also, I think knock back with bows should work in the same way, if it's fully charged you get full knock back, only half of the knock back if it's half charged and no knock back if under half charged. Maybe durability should go down faster if you don't charge a bow up. It's not very good for a bow's string to pull it back that fast.