How are you supposed to count your hair loss?

If a doctor is assessing whether or not your hair loss is normal and asks you to count how much you lose each day, how do you do this? I count all the hairs that I see in the shower and on my brush and stuff, but I know I lose more than what I actually see each day. So should I add any hairs to what I see and count to compensate for the hairs I might miss counting during a day?
Or is this a little obsessive?