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.
I’d imagine there is no technical issue with doing it live. The manual says you can make adjustments but it doesn’t say that construction is bad enough that the pots break in normal use. Unless you were ham-fisted, it is unfair to make you pay for the repair. I use a strobe disk and LED product you can find online.
Actually, they were very nice and said that they'd order a new factory motor and I would not have to pay. It would be covered under the warranty. I told them the truth that I really don't know if it was damaged to start with or if I caused it. I do know that the speed was not on specs originally, which was why I attempted the adjustment in the first place. My questions are about how I can avoid this issue in the future, once the warranty ends six months from now. Turning a screw should not be such a hassle. I've installed spark plugs in cars with no trouble and a turntable should be a lot easier!
Use a micro screwdriver and turn *very* gently. This is not an engine.
rff000 said "I used an RPM app on my phone and that seemed to work OK."

I never found the RPM app accurate. I’d get different readings depending how far I placed the phone from the plater spindle.
I read that the trick with those apps is to place an object around the spindle (like a roll of tape or plastic cup) and center the phone on that. When I did that I got a constant 33.9, but when I just laid it on the platter I was getting different speeds as it rotated. The 33.9 correlated with the mismatch in my digital recordings compared to the timings on record jackets. My digitals were always around 2% faster than the timings. I knew I needed a little less speed, but I didn’t know I’d mess up the screw and pot when I attempted the adjustment. Now it’s in the shop for a warranty replacement. Live and learn.
Since writing that two years ago I got my TD 203 turntable back from the shop long ago. It was still a little off so I gave it another try, turning the screws as gently as I could. The main problem was that each tiny turn made a relatively big change in the RPM, but I eventually got it as close as I could and I left it alone since then. If it had an easier adjustment I'd check it and readjust often, but now I don't want to even try since it's working OK and my warranty is long over. Not a bad sounding TT. I wish my digital sounded as good. Luckily I live near a used vinyl shop with very good selections on the one dollar table.