Try: 60 - seems to work ok, but keeps turning left (as it keeps going even when it's not longer registering light. We tried to stop the motors after making the sensor point forward again)
Stopping the motors in the end helped. But it seemed that the sensor wasn't turning enough when checking, so that the difference between the left and right light values was not large enough, whereby the motors were simply set to move forward, rather than following the light properly.
