in gen the CG would be apx 1/3 of the wing? from the leading edge?

Generally, 25 to 30% is a place to start. From there, we adjust for the kind of flying we like, or the plane is designed for.

Handyman,

That is an acceptable location for ground balance; however, the fine and final balance is completed during flight tests.

What you stated is only applicable to rectangular wings, for which the cord is constant for the whole span.

That is not so for tapered and elliptical wings, for which, a mean cord needs to be calculated and located over the real wing.
Then, the CG is located on that mean cord, also known as MAC (mean aerodynamic cord).

For such wings, the designer of a model normally provides the proper location of the CG to be measured from the leading edge back and next to the fuse.

Another important thing to keep in mind is that any balance should be done for the tail heaviest condition, which is when the fuel tank is empty, for a conventional model of wet power.

These two RCU articles explain it better:

http://www.rcuniverse.com/magazine/a...?article_id=84

http://www.rcuniverse.com/magazine/a...article_id=601

this works well for a fast and accurate start.....

http://sky.prohosting.com/air2/cg_calc.htm