I believe that pepakura measures the model, in your case, from the tip of the middle finger all the way to the top of the elbow piece. I think that is what might be throwing you off. I dont know how well you know the pep program, but after you unfold it there is a measuring tool you can use. You can pick two vertices and it will give you a measurement. It helps me with scaling alot as I can then adjust it. I would recommend to get rid of the hand and finger plates and then try it again you should get better results. If you dont then just measure from opening to opening on the forearm and then again at the openings to get an idea of the diameter of the openings.