Thread: weight distribution shank length?

i need a longer shank to bring the a$$ end of the truck back up again. the trailer is 13" to the bottom of the ball receiver and my current shank is 15" to the bottom of shank but 24" to the top of the ball.
how long of a shank would i need or how would i measure? i understand its the height of the trailer at level, minus the bottom of the hitch. whatever comes from that is the length you need, but being that the ball on these hitches sit higher from the bottom, how would i go about that?

thanks
robert