Actually, after educating myself on google, I think I only need a 2" rise, as the ball will add an additional 2". Please correct me if I am wrong, but from what you said it seems as though the height from the ground to the bottom of the coupler on the TM is around 17"-18", and with a 2" ball, that would bring it to 19"-20" to the top of the ball. So if I measured 16" to the top of my hitch, I need a 1"-2" rise for the ball mount.
16" to top of hitch + 2" rise + 2" ball = 20" to top of ball.
learning here!