Ignition timing is changed according to speed (centrifugal advance), or engine load, (vacuum advance). An "advance curve" is a graph, plotted with speed on the horizontal axis, and degrees of advance on the vertical axis. It's designed into the engine when it's built, by engineers with a lot more equipment and expertise than the average tinkerer has at his disposal. Unless you're well-versed in engine modification, and have a dynamometer and/or a distributor tester to check your results, LEAVE IT ALONE!

For high performance, or running a truck fully loaded all the time, etc., it's possible to tailor the advance for your specific situation. For everyday driving, just make sure everything is working right, not worn, and let the factory-designed advance system work the way it's intended to!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!