I have one of those plates and I seem to remember that it effectively shortened the slots on the inside? (Moves the weights out and then engages the plate).
So the total advance is still limited to the number of degrees labeled on the plate but it adds some tension so the curve shifts up.
Or I could be completely wrong don't feel like pulling the distributor and disassembling to find out.

Anyhow I doubt that cam will be happy idling at 600 rpm too. So if he's starting into the centrifugal advance at whatever the idle is, it will never be stable (more idle speed, more advance, more idle speed etc.... or less idle speed, less advance, less idle speed...)

My 451 (272@.050 mushroom) has a reasonably stable idle at 1050 rpm, 22 initial, 36 total, all in by 3000 or so. Could use even more initial but hot starter kickback begins around 24.
Only has 7" vacuum at idle - but it's manual everything.

Sounds like the OP should just start at a reasonable point like we suggested, and fine-tune from there!