As a previous owner of a company specializing in painting and drywall finishing, it sounds like you are off to a good understanding of the estimating process. A couple of variables to remember... calculations basically double if it may need two coats of paint, and always assume the worst case. If there are major color changes, or the existing paint is in really bad condition, two coats are probable. We always calculated about 250 SF per gallon for one coat, and if an additional coat is required, usually about 300 for it (second coat is easier, and goes further). Method of application is a major variable as well. The above calculations work if applying manually (brush & roll), but if spraying, lots of variables, including the type of equipment being used, and the experience of the person spraying. I hope this helps!