wkc

Well...I didn’t assume anything to start with. I just wrote down the distance travelled as shown on my bike computer and on MMR for each trip. That first list showed the BC recording bigger mileage for most trips than MMR. From that, I concluded that the wheel circumference in my BC was too big and thus reduced it for my second list.

The first list could have gone either way. I had no assumptions at the very start. I was curious as to what each recorded and whether or not there was a “consistent” error. And after a dozen rides, the BC was consistently showing higher/bigger distances than MMR.

My BC wheel circumference was set to 2150 at the start.

Observing the consistent overstatements by BC, I reduced my BC wheel circumference to 2110. If I were to use the actual difference of 4%, I should’ve reset my BC circumference to 2065 (after rounding). But as I wanted to “creep up” on the accuracy (in case something was amiss), I did not reduce BC circumference as aggressively.
2150 x (1-0.04) = 2064

Having reduced my BC to 2110, I am still getting BC distances greater than shown on MMR. But on this current/second list, the difference is averaging only 0.74%. This suggests I should reduce my BC circumference down to 2094 (or more likely 2095 or 2100) and start a new/third list.
2110 x (1-0.0074) = 2094

I have to admit that my main assumption is that a “properly calibrated” physical device on a physical rotating wheel on a physical road surface, will always be more accurate than a “thin air” device pointing to some satellite or three floating thousands of km/mi above the physical space being measured. Besides, it is generally accepted that phone GPS is no where near the accuracy of military grade GPS. (Please refer to above posts for discussion about accuracy or otherwise of GPS devices; which I do not believe helps in calibrating my BC. )

(edited for typos)