The only way to be certain of the ideal spot for a sliding fifth wheel (assuming total length is within and legs lights suzies etc clear without issue an rough ground ) is to load the vehicle fully, preferably with the tractor as near max weight as possible, and use a weighbridge to work out the individual axle weights, an axle weigher is ideal but if you note the total vehicle weight every time your drive each one of the three tractor axles off the bridge, then, assuming the weighbridge is level to the ground, you will have three reasonably accurate axle weights.
Note, on some 3 axle tractors, moving the fifth wheel back actually increases the weight on the mid lifter, no its doesn’t make any sense but it works like this on some, so a bit of trial and error is sometimes called for.
No most people can’t be arsed, i do this (but use an axle weigher) when i fist get a new to me tractor unit which is going to be my regular steed, so i know that A i’m well within legally, and B how the vehicle behaves and handles with the fifth wheel in different positions, and the answer is quite a bit differently, eventually you end up with a happy medium.