Any timing experts out there?

Not sure how useful to you this is, but vac advance normally works against a spring. Springs behave in according to f=-kx

(k=constant, x= displacement, f=force) - (http://www.pa.msu.edu/courses/1998spring/ISP209/homework/hmwk3/hwset3.html)

Consequently there would be a linear change in advance in proprtion to change in force applied by the vaccuum (i.e. negative pressure, i.e. reduction in force).

So once I get to the point - take what would be a normal vacuum advance maximum setting, and apply it in a linear fashion accross the vacuum delta. This should probably do the job until you find more solid info.

HTH