I use three marks to line up my 5th wheel.
Start with 5th wheel hitched pull forward in a straight line and stop.
Now put a mark dead center on the front of pin box that can be seen with the windshield rear view mirror.
Next a mark in the truck rear window dead center and at same height as the pin box mark.
Last a mark on the rear view mirror when the pin box, rear window and mirror marks are all lined up. This may require moving the marks on the pin box or rear window up or down to get them at the same height.
Now when backing just like aiming a gun when all three marks are lined up the kingpin should be dead center in the hitch.
It has worked every time for me. Even when I have had to approach the trailer at an angle I am close enough the pin will still go into the hitch pocket.