Almost painted ceiling

Applied 3 gallons (US gallon can is 3.78 liters) to the ceiling. That covered the whole ceiling once and most of it with two coats. I need about another half gallon so have to get another can of paint tomorrow. The colour is a Benjamin Moore Cloud White - it's a bit of a creamy white.

Three gallons should have been enough to paint two coats on the ceiling as it is about 480 sq ft. To coats makes 960 sq ft and each gallon is supposed to cover 350 sq ft, or 1,050 sq ft for three gallons.