Vacumn advance can adjustment screw, info please.

The rate is set by the spring rate and the total advance is set by the stroke. The pre-load or lack there of just sets the amount of vacuum it takes to start moving the diaphragm. The screw only adjusts the pre-load

I have had some ford vacuum advance cans that you could screw off the end and replace the spring and the stop thus adjusting the rate and the total amount, pre-load was set with a spacer under the spring.