There is a formula. It's called Equivalent Dihedral Angle. The idea is you can have a flat center panel with bent up tips or you could have multiple dihedral angles across the span or just one dihedral angle and they should all perform similarly. There is a little math involved and it's not very complex but I wrote a small program in MS Excel called "Sailplane Calc" along with an article for Radio Controlled Soaring Digest (RCSD) on how to use it. The formulas for tail sizing come from Dr. Mark Drela. He wrote an article in the August 2004 Issue of RCSD called "Basic sizing checks for homebrew RC thermal gliders".
In short he suggests an EDA of 6 for an aileron Thermal Duration model and an EDA of 12 for a polyhedral glider. It's not an exact science but these suggestions are a good place to start.
Here is the link to Dr. Drela Tail Sizing article:
http://www.rcsoaringdigest.com/pdfs/RCSD-2004/
and the link to my spreadsheet:
www.TailwindGliders.com
Curtis