To find out the exact number of turns you could use a Hall type of magnetic sensor. A small magnet is attached to the shaft and the sensor detects everytime it passes.
The challenge is to somehow permanently save it's position, I'd prefer some way of initialising the position to a 'start' setting at reboot/power loss instead of using the memory. I'd prefer some mechanical way to initialise it's start position. My guess is the hard stop on one end is the way to go, you can turn always towards the hard stop, then start counting from there.