^^^This.Generally speaking, isochronism is a physics phenomenon, which for any oscillator such as the balance wheel in a watch movement, can be affected by numerous different factors.
In the field of watchmaking though, the term isochronism seems to have taken on the more specific meaning of how well a movement can keep regular timing as the mainspring winds down.
Last night's exchange led me to reconfirm my own understanding of isochronism, as my understanding was solely related to watchmaking.
Indeed, as far as I can tell, my understanding was and is correct. All mechanical watches, even highly accurate COSC chronometers, will lose amplitude as they wind down, causing them to become less accurate as they lose power.
This is normal, and the phenomenon is generally called isochronism. It's not so much considered a "problem", only an important thing to understand in the context of discussing accuracy, particularly as it relates to how accuracy is affected by the power reserve.
It's interesting, and likely not coincidental that the STP runs within spec only when it's at least at half power, and the COSC testing requires full winding every 24 hours. The explanation I read for that is the chronometers are not expected to run within COSC spec at less than half power.
Unless I completely missed the point, I believe the OP was demonstrating how knowing the number of turns necessary to reach the varying states of power is useful, and again, it would be just as useful in a COSC chronometer. To know that number, someone at some point must have done the complex math our OP did in this thread.
New travel-sized Tapatalk is more discrete, and fits in your pocket without that embarrassing bulge.