There's surprisingly little information available on how to adjust the tension of the extruder idler. Mostly it's just "adjust until appropriate", whatever that means. One page mentioned that there's a long range of acceptable tensions, so I guess I will start with loose enough that it doesn't grip properly, then increase gradually until well after it starts gripping.
But first, I realized from looking at The Calculator that the Z steps/mm was a little off - it should be 2560 but was 2600. Not a huge difference, but might throw the layer height off a smidgeon. So I fixed that, which in turn made the Z offset wrong. So I have to re-calibrate that. 19x10mm first, then to reach the 10.43mm hex bit took another 4x1mm plus 5x0.1, so total 194.4 + 10.43 = 204.8, 3 millimeter higher than before (then - 0.2 to allow for hotend expansion, according to nophead). No wonder that test print curled up on itself.
There was a rumbling sound when homing X+Y, I should probably recheck the fastening of the gears if I see any sign of X or Y slippage.
The result is good, seemed to have slight initial sticking problems, still has a layer or two with gappiness and the top is still not fully closed. I will try slowly increasing amount of extruded material. (Cube #18)
Reducing my fudge factor to 50.0 / 30.0 and increased height to 204.7. Decent print, still with some adhesion problems, but the top looks a little better. (Cube #19)
Reducing to 50/27 and increasing to 204.8. (Cube #20). Adhesion pretty good now, but worth adjusting once more. Also less gap at the top infill.
Trying one more adjust: 50/25, 204.9. Now it's starting to get rising corners and thick layers. Somehow I manage to get overextrusion and underextrusion at the same time:(