So I went back to Danny D's dollar bill scene with Kiki Minaj, and found two usable frames, i.e. where the bill is exactly halfway down the cock*.
Unfortunately she's got her mouth on the glans and the NBP starting point isn't visible, so there's still some guess work needed, although there are some frames elsewhere where the NBP starting point can be seen, and the shape of the glans can be easily recreated. I decided to be extra generous at that end, to include the possibility that she's slightly squeezing or pushing on the glans.
If you think the NBP starting point I chose is unfair, keep in mind that the pics are rotated to make counting the pixels easier, and that throws off the angles and perspective, so please tilt your head to look at them before you speak.
View attachment 4578111 View attachment 4578131
Results are 8.19" and 8.18". If we strictly follow the shape of the glans to estimate the tip end, we get 8.12" for both pics.
* The base being closer to the camera than the tip, doing it on frames where the bill is closer to the base would shortchange him, and closer to the tip would be the opposite. It can be argued that for the same reason, the true mid-point would then be just past the pixel mid-point, but unfortunately there are no such frames, as she moves the bill up and down the shaft quite fast.
In frames where the bill is undoubtly closer to the tip, it ends up 1 pixel shorter, so if we decided to make it 0.5 pixel shorter to try and compensate for the perspective, we'd get 8.24" and 8.23", or 8.17" for both pics without the glans squeezing compensation.