First disconnect the vacuum line at the distributor, if it has one, and plug the line. Next idle the engine down to 700-800 rpm to make sure the centrifugal advance is not operating. Next read the timing with the light, this is your initial timing (do not adjust anything). Next start to speed up the engine and record at what rpm the timing starts to advance and at what rpm it stops advancing, you may have to go above 4000 rpm. Lastly what is the timing when the ignition stops advancing, this is your maximum centrifugal advance and again what was the RPM at this maximum.
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That is an excellent, concise, explanation. I am sure this "how-to" will help many folks. (myself included).
Thanks,
Pat