"The difference in where the limiter is (in front or behind like the Crane), has NO effect on how much vacuum it takes to first move the vacuum advance rod on the can I'm testing right now".
I am quite sure that changing the springs nominal position or position without any vacuum will effect the point at which it starts to move due to a vacuum force applied to the diaphragm. The limiter plate on the back side as I stated will push against the rod which in turn will put some preload on the spring. This should effectively raise the pressure that is needed to begin to move the rod. However I am not the one doing this with a dist so I don't doubt your findings cobalt but I will try it myself. This is a cut from the crane instruction sheet that states that changing the springs nominal position with the limiter plate at the back of the rod may require an adjustment to the cans spring set screw in-ward or clockwise which on my dist anyway causes the vacuum to come in sooner, so the nominal spring pressure must have an effect on the start point of the movement...or so it seems "intuitively"
As vacuum timing is decreased it will automatically increase tension on the vacuum diaphragm spring. It may be necessary to decrease tension
by turning the allen screw adjuster inside the vacuum port of the canister clockwise.