Thank you for this WONDERFUL post. I spent 30 minutes retightening all my belts this morning, feel a lot better knowing there is some science and calculations behind my belt setup now.
For anyone else following down this path, here some additional info I found.
On my older iPhone SE, the measurements were (occasionally) all over the place. I kept at it until I got 4 or 5 nearly identical measurements. I also found that where I positioned the phone (in reference to the phone’s microphone) effected the results. So be consistent.
I initially was strumming the belt with my thumb. Then discovered I got more consistent readings if I used two fingers to actually lift the belt and then let go.