When in doubt, start with 25%. It might land hot, but it will fly without giving you a heart attack.

Here's the ones I know of where serious CG mistakes were made:

The Hangar 9 Camel. 4" from the upper LE gives you over 30% of MAC. Not good for an 11% of wing area stab.
Mikulasko's Arrow. (Plans) They say to balance with the tank empty, rear mounted tank. Guaranteed control problems.
Lanier Mariner 40. The CG measurement for the 120 size is used in the book.
Fun Aero Albatross DIII. Way tail heavy.