It's just geometry and will vary by hitch brand and track model. Assuming there is a 6-8 inch clearance between the top of the truck bed and the bottom of the fifth wheel, then you can calculate the angle from the hitch pivot point to the far end of the top of the truck bed.
You have two sides of the triangle, the clearance is one and the distance from the pivot point to the end of the bed is the other. You'll need to measure the distance from the bottom of the fiver over the pivot to the end of the truck bed.It will be an obtuse triangle and given the three sides you can calculate the angles.