Re: How much bottom paint?
Using multi-season paint on a 30 ft hull: I buy two gallons. Two coats, then use the remaining 1/2 gallon or so to put an additional coat or two on the water line, leading edge of the hull/keel and rudder.
This schedule worked well for me:
Spring launch with fresh paint
I have the bottom scrubbed in the fall by a diver
Winter wet storage
Have the diver clean the bottom again in the spring
Haul out in the fall for dry storage and a paint job in the spring
I always thought I could get another few months out of the remaining paint, but, didn't want to risk it.