I would think the main thing is to KNOW WHERE the turntable is at and not just count pulses so you would also need a reference signal (pulse) to let the program have a known position.
As far as having an encoder being driven by the turntable over time there would rolling errors due to the mechanism not being a true gear train ( 300 teeth/30 teeth) but rather a not very accurate system ( a 10.0xxxx inch rolling another 1.00xxx wheel).
If you don't care at what position the turntable is at then why even have an encoder?