Hello, I just finished my 4* 120. It weighs 11.5 lbs. When inverted on the CG machine, it will have a tendency to dent the sheeting. Before sheeting, I first make sure that the CG range, or area that will have contact, has support under the sheeting. Usually the spar is there... but if not..... I put a small block of balsa between the sheeting, or whatever I have to do to give it support. When I have the wing sheeted and FINAL sanding is done..... I apply some thin CA to the "general area" on the sheeting where the CG machine will touch the wing. This will harden the balsa so it minimizes any denting.
After covering.... I cut out some black covering about 1" long X 1/8" wide. I apply these small strips neatly at my choice for the CG. (Usually my CG point is about 90% forward in the range...I am starting my third year of flying and and am still fairly conservative!!)
I am not sure exactly how the GP ..CG machine looks at the balance point. All I know is mine does dent a wing!!! (it did it on my ARF Spacewalker). It is a homemade deal... kind of copied from the GP machine. (except for the contact point). I used aluminum arrow shafts for uprights, with rubber tips slipped on the ends that contact the wing.
It works great......so far...!! My Spacewalker at 9.5 lbs is the most weight I have put on it so far. The 4* at 11.5 lbs will be a test to it's worth!! .........lownslo