what I usually do in cases like this, is the following:
1. measure your wingspan an divide it by 2 -> length l
2. from the wing centre go to 0.4*l
3. take 28% of t (profile length at 0.4*l)
4. your CG will be at this point.

This works as long as the wing has a nearly elliptical shape (->spitfire). This CG is a good starting point. If the wing shape shouldn't be exactly spitfire like, the plane will be slightly nose-heavy. I never encountered any tail-heavy configuration with this method.
Have a look at the drawing too (not the best one I made..:-) )

Hope this helps you