You basically want to set up the fifth wheel ahead of the rear axle. To properly calculate you would determine the maximum trailer weight and then calculate what your front and rear axle weights would be accordingly. Seeing as you have a slider I would set it so that the kingpin could, at the furthest back, be directly over the rear axle, this way you will have the maximum range of the slider, and there is no case where you would want the kingpin behind the rear axle.

You also have to consider the fifth wheel height. The plate should be between 47-48" above ground so that the trailer will sit level. You may need to add a sub-sill below the fifth wheel plate to bring it to the right height if you are too low. In an old International manual I have it says to run the sub-sill to 1" behind the cab, and not to use a wooden sub-sill less than 4" thick. If you need a shorter one it says to use angle iron.

Tad

1946 Chevy OS 1.5ton, 1955 GMC 3100, along with other various IH,Ford,Nash,MGB,.....