Minimum Paint Pricing

Very new here 2nd day/ 2nd post. When doing a proposal have a min. product cost. If I am only painting baseboard in a 12×12 room and a gallon of paint cost $40 and the coverage rate has it using $12 of paint it should still estimate the product cost at $40 cause SW and Ben Moore are not going to give me my other $28 back lol. Its a time suck to have to check every substrate to make sure it has figured enough. Thanks you guys are doing great so far keep up the good work.