I assume this is for belt driven turntables, not direct drive. I also have a pair of 1200's. Most mixers these days have BPM built in, and before that you had red micro bpm counters and headphone amps to validate BPM. Never once on my 1200's did it read something outside of the producers BPM count when pitch was locked.
My MK2 was off by about 3% when the pitch adjust was in the detent. To be fair, it had a rough life before it ended up in my hands. If you have a frequency counter they're pretty easy to calibrate the correct way though.
Did the delta show up on the strobe? Or is issue that the quartz is off and it’s feeding both the motor and the strobe? (Sorry if this is a dumb question. I don’t recall seeing speed calibration in the mk2 manual when I was setting up my secondhand decks)
I honestly can't remember if it showed up in the strobe or not, this was a few years ago. This procedure wouldn't be listed in the regular manual. It involves opening it up to adjust an internal potentiometer, and can't be reliably done without a frequency counter (I guess you could do it through trial and error, but it would take forever having to reassemble it and play a test disc after every adjustment,) so it is only in the service manual. You should be able to find a scan pretty easily though. Can't remember what frequency you need to hit, but it's low. If you don't have a proper frequency counter an Arduino will work just fine.