4.75 - 4.77 millivolts is the max you grind it if memory serves me correct. If you go over it, you'll throw a DTC.
The first thing I'd like you to check before you start grinding away at the stop is to open up the throttle all the way first. If the blade is perfectly flat when open, don't grind. Otherwise, grinding it down will cause the blade to over open, and act just the same as if it weren't opening enough no matter what voltage signal you get off of it.
The premise of this mod was to get 100% throttle opening out of it by grinding the stop. Generally the PCM reads 100% throttle as a millivolt signal of 4.75 - 4.77 as I mentioned above. So accordingly if you're getting 4.2 millivolts, it may be only opening 92%. But without a visual inspection to go along with this, you can't make a determination as to the accuracy of it's reading. For instance, if for some reason yours does read 4.2 millivolts, but after visual inspection, you notice that the blade is in fact 100% open (I.E. flat), then there is no need to grind no matter what it reads.
Doing this also won't net you any large gains though. It's more of an operational type thing.