I haven't seen the vid--yet--but as the previous poster said, yeah, you're supposed to measure down the top.
HOWEVER, if you're going to use a still camera set up to one side for this next movie as it seems you did in the previous ones, it'd probably show better if you measured down the side, with the tape actually facing the camera.
As far as girth, my suggestion would be to measure in the middle and again wherever he happens to be the fattest if it's not the middle.
Getting measured is so hot to me. I've only experienced it twice, most recently just a few weeks ago by my wife. She calls me big all the time, but her actually verifying it right after reading in Cosmo that average length was between 4(?!) and 6 inches was just awesome. I was some kind of horny for days afterwards.