How much filler to mix for the areas you're repairing, and how much primer to buy for the film build you want. The primer math uses the same physics paint makers use on their tech sheets.
One gallon of pure solids spread 1 mil (0.001 in) thick covers 1,604 square feet. That's simple geometry: 231 cubic inches in a gallon, divided by 0.001 in, divided by 144 square inches per square foot. Real primer is part solvent (volume solids tells you how much is left once it dries), and not all of the spray reaches the panel (transfer efficiency).
A quart holds 57.75 cubic inches. Filler is sanded down heavily after it cures, so the waste allowance is larger than for paint.