As in the length questions (where to measure, how to measure) consistency is the key to recording gains. If you have a full hard erection and put “normal” (whatever that is) tension on the tape then you should have only the skin to compress if you pulled it tighter. Find a way that’s comfortable and easy to do and do it the same each time. If that means getting the tape measure as tight as possible, then that’s what you’d have to do each time to get a consistent measurement. Having the hardest erection will also help prevent compression problems.
The corpora cavernosa (CC) do contain tiny smooth muscle strands, but they do not “fill up” the CC. Blood is how an erection is created and maintained. The muscle fibers help contract the CC and move out the erectile blood when the penis goes flaccid.