That's probably because a chart can't tell you how far from the receiver the rear axle is and how far from the rear the front is. You can do the math but it's going to be a guess at best. Personally I wouldn't trust a chart even if you found it. Obviously an extension from one truck places the ball at a different point when compared to another truck with a different overhang.I can't find definite info on how tow ratings change with so much payload and an extension. Any charts?
https://en.wikipedia.org/wiki/Torque