Flaccid length is a temporary thing as in most cases you will get flaccid length changes before you get erect length change. I will consistently have a 3/4” longer stretched flaccid then Erect. I Check my stretched flaccid length while hanging SO. A very consistent and accurate measurement.
Matter fact: I don’t worry very much about my erect length any more I just shoot for more stretched flaccid length and relax in the knowledge the my erect will catch up when it’s ready.
The true measure of exact length without variables is pumped length. Put measurement marks on the outside of a pump tube and you will have the most accurate measurement there is. There’s no cheating in a tube.