The manufacturer's rep can tell you the optimum film thickness for that paint
. Then see if he'll let you know where to get a gauge to test the thickness of your paint after you've applied the full 3 qts.
We usually use the left over bit of paint to coat the high wear areas: the waterline, and the leading edges of keel
I'm not sure it's correct, but we have been told that it's best to use paint of the same age, like to not mix last years and this years. Of course, I can see how this concept
might benefit the sellers, so don't really know if it is true.