Terminal Velocity Estimate
Under open-loop control the terminal velocity is an indicator of the friction in the turntable. Thus, measuring the terminal velocity is a good way to quantify efforts in friction reduction.
Below is a method written in pseudo code to estimate the terminal velocity
Turn on the motor at full speed
Measure the pot value and StartTime = millis().
Wait a set amount of time (you may have to experiment to find a good time delay).
Measure the pot value and EndTime = millis().
Serial print the ElapsedTime = EndTime - StartTime
You will have to watch the screen to see when the Elapsed Time settle done. You will see erratic jumps when the pot goes through the dead zone.
Note, in this example we use delays and serial prints, which can be detrimental during real-time closed loop control (but is fine here).