Generally two different checks...total of centrifugal and static timing is checked for full-throttle operation with vac can disconnected, vac adds in more advance for general cruise conditions so engine isn't retarded during most driving. Going in, check your static timing as in shop manual or whatever spec you use. Find out roughly how much centrifugal you have by twisting rotor (on most engines) and marking the arc on distrib, then multiply by 2 to make crank degrees. Try to find both factory curve (how many degrees at what RPM, centrifugal only) and some generic hotrod info on your engine for parameters. Disconnect and plug vac, rev up engine with timing light and degrees marked off on pulley. See what happens whe, and when total is achieved. Most engines like full (full throttle) advance by 2,000, most have advance too slow in stock form.
What year and type distributor? Does it have that year's vacuum advance unit on it? Points distributors up to early 70's ran about 24 degrees of mechanical advance and the vacuum added about 10-12. Setting these at around 11BTC at a 650RPM idle with with the vacuum disconnected was a good ballpark. Later "smog distributors" ran less mechanical advance and more vacuum advance for emissions reasons (but they sucked in the power dept.) Some of them ran little or no initial advance. All of this is assuming you are running a manifold vacuum vacuum advance, the way Chevy designed it. I like to run a vacuum advance that advances 10-12 degrees and mark my dampener at 36 degrees, on a '69 or older points distributor then with the vacuum disconnected I run the RPM's up till it's not advancing on the mechanical centrifugal advance any more and I crank it in so that 36 degree mark is under the zero pointer and clamp it down. then I reconnect the manifold sourced vacuum advance. Sometimes on our "winter blend" I have to kick it back a couple of degrees from that to keep it from pinking when cruising at high RPM (Autobahn speed)
In general, the timing is the precise moment of when the spark fires compared to where the piston is in it's cycle. As the rotation of the motor speeds up, the timing needs to advance in sync. Your total timing is your initial timing set (checked at idle) with the additional advance that the rpm's bring dependent on your system (checked at high rpm).
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