Is it better to construct the spar at 25% mac or at the cg. on my 3D planes the cg is quite aft so should my spar be at the CG.
fnp

If you are using an airfoil, then put the spar on the high point of the wing. This is where all the forces happen, regardless of the CG location.

If you are using a Flat plate wing, then go with 33% MAC for the spar location. It's a good spot for most applications. Wing sweep may be a factor...

by highest point do you mean where the airfoil has maximum thickness. In some airfoils the max thickness is different from the highest point like in a semi symmetrical for example.

In referance to the upper surface in normal flight attitude. Some of the HG, Eppler, and Selig airfoils use a forward transition point on the lower surface...

If it is symetrical (Assumed for 3D/Aerobatics) then the thickest point should also be the transition point for the upper and lower surfaces.

If you are still ripping wings off... better let me help out on designing some new ones