Hey, look what I found.

http://members.cox.net/evdesign/pages/database.html

OK so the table of design info isn't readable (unless you guess a lot) But the chart of vertical tail volume to dihedral angles does show some relation. It's a grasp but if you imagine a straight line through the median it does rise with the increase in dihedral.

And here is a site that has the formulas for both the volume calculations. It's gratifying to my testing results that the center of gravity is used for calculating the vertical volume as well as the horizontal.

http://www.eaa62.org/technotes/tail.htm

I feel vindicated......

Gotta go look for spiral instability nowl..... I'll be back.