Generalization is correct. Heat loss is based on the characteristics of the home, such as # outside walls, windows, doors, attic above, cellar below....ect..
Sounds like a lot of variables, but not that bad to tally. Outside Temp. is the big factor, use too low of a number and you over estimate, too low under estimate, zero is a safe bet. To base a calc. on your general formula would be a wasted exercise, too many variables not calculated in.