It gets tricky because then you get the whole "bone-pressed" debate.
You measure from the top of you penis out, where the penis meets the body. A tape measure is best because some penises curve and will give a shorter measurement with a straight ruler.
Bone pressed means you press the ruler into your body to your pubic bone. This is supposed to give the most consistent measurement. The problem is that it can give you about a 1/2 inch to an inch of extra length that is not really visable. Some say you should only measure what you can see.
So pick a method and lie like everyone else.