Well, its going to take some physics and some trigonometry to calculate it accurately. Basically, make a triangle with your arms, your body, and the floor. You're going to need to know the distance from your toes to your shoulders and your body weight to calculate it. I don't feel like doing the math... damn I'm a dork.
Just get down in the front leaning rest and put your hands on your scale. It will show you the amount of weight. Then compute it based on your actual weight.