I zeroed my M&P15 with the handle at 50 yards, just like the M4 and found it would not zero with the front sight in a useable position. Perhaps at 25 yards it'll work, but I ended up having to adjust the rear sight to bring the rounds up onto target. Would have been nice to use the IBSZ settings, but it didn't work on my rifle.
The range markings are useless on the M&P15-22. I prefer an "all the way down" rear sight battlesight zero, though, rather than counting clicks.
There are two handle heights because there are two front sight heights. F-marked front sight bases as used on the M4 Carbine are shorter than unmarked bases used on the M16 and many (most?) AR15s. Shorter F-front sight base requires a shorter rear sight base to properly zero. Or a shorter height handle. This is why some handles work and some don't.