I shoot one photo on the first of each month: same lamp, same spot, no flex. Bathroom glass says nothing has changed. Phone gallery says my jaw and shoulder line have. I trust neither as ground truth. The mirror has live data but terrible noise: salt, sleep, posture, mood.
The photo is frozen but also noisy: angle, light, whether I remembered to exhale. So I treat them like two sensors and check a third.
Waist tape at the navel, first thing, before coffee. That number is boring, which is why I believe it most.
When all three disagree, I log the spread and wait for next month. Anyone else keep a tiebreaker metric for this?