Until I got the Great Planes CG Balancer, I was at best guessing with my fingertips at the cg mark under the wings. The unit comes in parts and it wasn't hard to assemble. The directions weren't clear on the placement of the balancing pad area but a quick look at the pictures on the package box cleared the mystery.
Someone clever figured out the simple but effective CG mechanism and the resting pad area for the wings are about the size of a nickle. Using this balancer, I found out I needed 0.4 OZ (two 1/4 inch screws weight) at the tail of my Apprentice 15E with Turnigy Nano Tech 3000mAh LIPO for proper CG.
By using this balancer I am able to determine proper battery placement or need for counter weights depending on the battery capacity (eg.using 3S vs 4S, or lower/higher mAH) on all my planes.