The tricky thing about formulas like this is that it is very domain dependent.
What you describe is a reasonable approach for a freelancer who expects to bill most working hours. It falls apart for a lot of consulting scenarios where you bill fewer hours and spend more time generating work. In that case you may be better off setting rates so that e.g. 1000 billed hours will reach your base target salary equivalent...
I did say the formula would need to be tweaked as needed ...
OTOH, this is more or less how clients themselves expect to be billed so if you deviate too much without any logical explanation, they will simply go elsewhere.
What you describe is a reasonable approach for a freelancer who expects to bill most working hours. It falls apart for a lot of consulting scenarios where you bill fewer hours and spend more time generating work. In that case you may be better off setting rates so that e.g. 1000 billed hours will reach your base target salary equivalent...