Just take something to mark the existing location of the 4 bolts. Draw around them with a sharpie or paint pen. This way you know for sure how much you've moved the box, and this ensures the rails are still even. You know you need 1/2", or whatever, then there's the starting point to see that much movement at the bolts. It'll get you close.
It's easiest to move with the slide partially extended, or retracted. There's a sweet spot where it's easier to move kind of balanced on the Teflon wear strip.
John and Rebecca Dickson
Emma-8 / Little John-5 / Iva-1+
2013 Ford F-350 Lariat FX4, CC LB PSD, DRW
2015 Jayco Jay Flight 28BHBE (#8)