Recommended position for sliding 5th wheel

Its not so much which hole.

If you have a 48 foot flat, a 53 foot Van or reefer etc, what I do is set that 5th wheel where the front wall of your trailer (Not including reefer unit) ends just in front of your drive tires on the first axle generally.

If you took a ruler that is really big on the ground against the first tractor drive tires in the front and put the front of your trailers where the glad hands are, up against that ruler up and down you should be very close if not at that ideal spot.

Spray that spot with a neon paint of your favorite color so you can put that 5th wheel right back to that particular slot at any time with a pry bar before hooking on.