The manufacturer's site is the best plaice to go for the proper
installation instructions.

Its a simple lever - you want some weight on the front axle so you
need to place it somewhat ahead of the rear axle, but not much.  As
others have advised you can get the mfr recommendation but under no
circumstances would I install it directly over the rear axle, no
matter what the mfr said.  When you actually get down to installing it
you will discover that reality gets in the way of the theory anyway.
Little details like fuel lines, brake lines, spring hangers and cross
members will greatly limit where you can actually place the brackets.
In most cases there is a very small range of positions where the
brackets will easily fit against the frame and that will determine the
exact final location of the hitch.  My goal would be to have the
centre of the kingpin about 4" ahead of the axle centreline on a
diesel and maybe a little more if you have a gasser.

But that's free advice and worth exactly what you paid for it.

If you want to get really fancy you can do the math.  You've got two
lever arms - the distance ahead of the rear axle and the distance
behind the front axle.  The ratio between those two arms will
determine how much weight goes on each axle.  Assuming you know the
pin weight of the trailer and the current weight on each truck axle
you can make the determination of how much weight you are going to end
up with on each axle.