I had to overcome an error yesterday that highlights the importance of isochronism. I was adjusting a 992 to position and had it within 4 seconds across the vertical positions. But the horizontal positions were about 20 seconds slower. After some thought, I looked at the regulator pins. Very close spacing. But then I realized that I had picked up the watch from the day before and had not verified isochronism. There was about 90 seconds difference between 180 and 270 amplitude. I readjusted the pins to provide a spring width space on either side. This made the watch 120 seconds slow in the horizontal but reduced the isochronal error to 10 seconds. Turned in the timing screws to bring it to 10 seconds fast from the 120 seconds slow. Then tested positions at 270 with a delta of 7 seconds across all positions. This illustrates two things. That isochronism is primarily independent of all the other adjustments and must be checked. Secondly, the oft written statement about keeping the regulator pins close is to be viewed with skepticism. As far as I know, most of them did not even have the watchmaker check the isochronal rates. Kind of like telling people to perform static poise without first checking the positional rates. And unless the staff is obviously damaged, I recommend checking the watch performance before doing any service.