I would put your penis into a position so it is at a right angle to your body. This is possible as it is how you give head. the measure. I would personally say the top side of your penis, as the magic inches you can create on the under side of your cock while it is upright are not inches of cock anyone can use! if you put your penis at a right angle that's pretty much all the penis you can use and is more the kind of position it'd be in while you have sex.