A tape is not rigid. Thusly, contours of the penis, like a thick corona ridge or large veins, would take up a small portion of the tape. Those wrinkles in the tape translate to extra length in this measurement system. It is why length is most repeatably measured with a ruler. Tapes are also not created equal and if you use a tape for girth, use the same tape every time to track results better.
The way you measure is not to get the biggest numbers. But to get the most repeatable numbers to track progress. Now, with you establishing how you’re going to measure you’ll establish a baseline. So establish a strong, repeatable baseline…whatever way you want. We aren’t here to judge. However, whatever way you establish that baseline, measure that way from here on out to accurately track progress.
Even if you want to measure hanging upside-down from a chin-up bar.