multi turn potentiometers to measure degrees
First of all, it's kinda bad idea - potentiometers have noise while in movement, and this will distract your readings. Proper solution is to use encoder - they already have digital output and way way more reliable.

For multiturn potentiometers you just have to calibrate them: rotate by 90' write down ADC reading, then again and again until you reach the limit. Then you do interpolation to get reading between your calibration marks. In order to reset to 0 - you just add %360 in your code (so that calculated value would 'wrap' after 360').