Problem with Thorens TD-203 speed adjustment screws

My Thorens TD-203 was rotating around 2% too fast (33.9 RPM) a few months ago, so I tried to adjust the little adjustment screw on the bottom of the table. I wound up getting no speed change but apparently ruining the screw. I just brought it in to the store's service department because it's still under warranty and they said the potentiometer was broken and the motor has to be replaced. I saw some videos that showed the adjustment done "live"--with the platter rotating. I tried to adjust the screw with the power off and by tipping the table up. I later found some messages that said this adjustment is very tricky and breaking the potentiometer is common.

Is there any advice about this other than never attempt a speed adjustment? Do you have to do it in real time or can you turn the screw a little bit and measure RPM after that? I used an RPM app on my phone and that seemed to work OK.