The thing with the slots I call the 'slots'or athe 'plate' but IIRC Chrysler calls the governer.
I can't guess what the number might be on it but probably not 30.
9, 11, 13, 15 range are the most common. Some will be R others L, some with cut slanted which effects the rate of advance.
Anyway those are the maximum degrees of distributor advance. Double that for the degrees of advance on the engine. With an electronic distributor particularly, assume in actual use a few degrees less.
The vac arm you interpret correctly.
How To Limit and Adjust Chrysler Vacuum Advance Cans
To see the effect of changes without a distributor machine, use the engine, a timing light, and a tachometer.
measure and write down the timing from as low of rpm as you and then at a number of points at higher rpm.
I like pictures so I plot the points on a graph.
it's possible to ballpark changes by making a pointer and a degree wheel.
I think that 60 psi cold and 25psi hot at 650 rpm is great. As long as the pressure rises back to 60psi on the highway I'd stay with your current viscosity grade whatever it is.