Runners agree that the formula 220-age can be way off for some runners, but what's the biggest deviation from the formula you've seen? Have you seen it be off by more than 39 beats?
At age 50, my MHR of 191, measured by the UC Davis Performance Lab and confirmed many times in 5k races, gave me a deviation of 21 beats. Yesterday my out-of-shape hard interval repeats gave me a high reading of 186. And no, it's not optical. This is with a chest strap. I hit 184 last year doing a long hill time trial so 186 is a realistic MHR for me.
220-age (73) predicts a MHR of 147 for me so the formula is off by 39 beats. Have you heard of anyone with a higher deviation than that?