Main source of the problem is that the nut well on the large gear is 1 - 1.5 mm deeper than the nut thickness. So the jam nut cannot make contact with the nut in the well. I noticed this during assembly but failed to act on it then.

The two M8 nuts in my kit were both thin nuts. The well in the large gear is sized for a standard M8. Fortunately, I had a standard M8 in a spare parts box. When I swapped out the standard nut for the thin nut, there was enough projection outside the large gear for the jam nut to do its job.

Estep calibration complete now at 880, with fairly consistent extrusion lengths. I am cautiously calling this a solved problem.

Thanks again for the help!

Jim