I'll assume you already solved this problem, but, if you didn't....
The Fanuc manuals claim that the units for Grid Shift (1850), and Backlash (1851) are in microns. Over the 20+ years I've been repairing CNCs, this has rarely been the case. I typically make an adjustment of 25 or 50, record the amount of movement, then calculate how much more I need to adjust from there. With backlash, you can simply put in a number and decide how much to increase without calculating anything, since it's not very exact.
The rule of thumb when I worked at Yamazen (which I still follow today) was; If you have a value greater than 100 in PRM 1851, it's time to find the mechanical issue.