Vacuum Advance Question

Vacuum marked on canister is dizzy degrees. 8.5 is 17* at the crank, 11 is 22*, etc.
Adjusting screw changes inches of vacuum to start vacuum curve, CCW less CW more. To limit the total amount of vacuum advance 48* to 52* you can bend the canister housing with a flat blade screwdriver outward. Make sure the canister your using is not leaking.

First thing if you haven't already done it is install a timing tape or mark the balancer so you can see what timing really is. I do not use a dial back timing light, a standard light works best.
Second, Set timing to 25* initial (you may have to go up or down depending on what the motor wants) and get the idle mixture set with the transfer slot squared up. You want to get idle mixture and timing dialed in for the smoothest idle/highest vacuum. Once you determine the initial timing you can now setup the mechanical curve.
My mechanical is 35* total with 9.5:1 93 octane. If I add more it pings under heavy load. Your motor maybe different, you'll need to experiment and see what the engine likes. I'm using an old dizzy that I welded the slots to limit advance. If its to much reduce the slot. Do the mechanical curve with vacuum advance plugged.
After you get the mechanical setup correctly you can now play with the vacuum advance to get cruise and light load timing set for better gas mileage. The allen wrench adjustment adjusts the vacumm it takes to move the advance can. The amount of advance the can has it limited by the stop on the rod. You can lower the amount by bending the can up with a screwdriver. My vacuum at idle is 15" and my vacuum advance is all in at 15" ported. Take the car out and set as much vacuum advance as you can at cruise without pinging. Again you need to see what your motor wants.