As a seat of the pants technician, I had to learn to time a machine by trial and error. Mainly error. You just hit it on the sweet spot... Oh and here is a thing I put together a while back because we had so many people thinking their machine was out of timing.
http://www.quiltingboard.com/vintage...l-t147275.html If somebody knows more that that PLEASE add to it. Some of it may be gone or out of date.