You should always let your decisions be dictated by the easiest number to calculate precisely. If it's just a number somebody gave you, that doesn't seem very

You should always let your decisions be dictated by the easiest number to calculate precisely. If it's just a number somebody gave you, that doesn't seem very smart. If you have to guess and wave your hands, that doesn't seem very smart, either. If you compute the number using math, now that's smart. You can graph it, you can pick a nice sharp-looking number of decimal places. You can write the formula using algebra. That's economics.