The flaccid measuring is right on but the erect are way off. Dick should be paralel to the flour, ruler ON TOP CENTER DICK to tip.
The dude is measuring from the side and this leads to very inconsistent numbers each time you measure. From the top pressed against the pubic bone, on the other hand, gives you a fixed point for all measures, as the pubic bone won't change places.
So measure from top cause from the side you get an easy inch that is inner penis.
This is how i do it and is only my opinion. Others may not agree with it.
Peace
Mike