Cant see anything wrong with meassurements like that. If you put the ruler above the penis it's the same length still. You didnt put the ruler bellow your balls so it's an okay way to measure imo.
Some people got an upward bend, (me included) so I gotta press down my dick like you do and holding the ruler beside to meassure. Complicated stuff. But aslong as you keep track that you are gaining it doesnt matter, measure in one way and one way only to keep track.