Amount of advance is determined by the total length of the slots.*

Closing up the inside increases the starting tension of the light spring. (The heavy spring has no tension to start because of the long loop)

Closing up the outside stops the advance earlier.

The shape and rate of advance depends on the spring rates and when they come into play relative to rpm. That's why the side welded can matter.

*When it comes to figuring the advance, measure the slot and measure the pin diameter. The weight's movement is equal to the slot length minus the diameter. Divide that by the stamped number to get movement per distributor degree.

IIRC, 20 degrees is typical on a vac can. If you subtract the two numbers I posted earlier, that is the amount of vac that Mopar Performance was shooting for on their Electronic ignition and distributor kits. You can adjust it a bit with the allen screw as somone already posted. You can also modify the arm by notching the stops.

Added Note: The Mopar Performance instructions mentioned above use 2500-3000rpm as the range for total advance should be stable. YMMV.