IMO, maxpotapov is 100% correct. The work done by the legs and torso loads up the muscles of the lagging arm/shoulder, and this energy is released when the arm fires. I believe there are studies that show that stretched muscles are able to generate more force/speed when they contract. Ergo, factoring the legs/body out of the equation will have a big impact on the final power delivered.
Of course, the arm muscles are very active. My contention is that the more we are able to let the arm relax by using the legs/body, the more control we will get. If the arm muscles are 100% active, yeah, there will be enough power to launch the ball to the moon, but control will go down the toilet.
Also, IMO, there are many ways to hit the forehand (or backhand). This variation may be due to stylistic preferences, and/or game situations. Thus I would say that it is not realistic to assume that a particular muscle group always contributes a fixed percentage of power for a specific stroke such as the forehand.