For me i dont calculate like that, i do calculate how much time i would spend to solve the problem, and include the additional charges like petrol, toll, etc which incurs on me. Then I do use the maximum posible time for calculation to avoid any loses. IE If some1 ask me to develop a software, i would need to know pretty well the specs of the software he need and then i will think how many hours i need(max posible) and then multiple by how much i think my hourly salary my service worth(it depents on the market, if uses technology that seldom ppl has, it has to be higher)