Engine timing
 

I have a W123 (115 engine) and just out of curiosity I tried to vary the distributor advance without the use of a timing light and I noticed that if you move in the counterclockwise direction the RPM goes up to a certain point. I believe this advances the timing while clockwise would retard it. I know this is not an accurate way of correcting the timing but would it be safe to assume that if I am within the correct range, that portion is where I have the relatively high RPM range? Pls. advise. Thanks.