[RyanN]

To add to Ether's recommendation.

I was teaching how to use encoders and applying geometry and trigonometry. One of the tasks I had them figure out is to make the robot turn a certain angle using left and right encoders.

They were successful, but due to the slip-n-slide nature of your drive train, it really is by chance (aka lottery and moon phase). It was within about 10˚ of where it should have been.

Of course, a better solution (as also mentioned) would be to use the included gyro. It's easy to wire (have a spare PWM Cable?), easy to program (They have an example on how to use it), and easy to use (output is already in degrees).