Hi Roxy. My firmware already has the M851 command coded.

I put m851 in pronterface and it shows the same offset on my LCD screen. Then I type"M851 Z-13.91" to go from my current offset of 13.5 to the new offset of 13.91mm. All this while the printer was paused, I then resumed print and I am unsure if it accepted this new offset or not. I dont think it did, was my syntax correct in pronterface to issue the change?