Partially Solved Z axis setting (M565): Correct now?

Over in this thread, I got some advice from mark tomlinson on adjusting my Z-axis value via the M565 g-code command. Thanks for that! I've now managed to run some tests, and I think that a value of -1.39 is about right; however, if I understand correctly, it's showing signs of being both too close and too far -- I just hope not too much of either. Here's a photo:



Near the edges of the circle, it appears that the extruder has gouged into the filament a bit, but near the center, the filament looks better, but maybe the filaments are a little too cylindrical near the center of the test print. Clearly, the discrepancy is caused by the fact that there's some overlap between the outer circle and the back-and-forth lines filling it in. As I'm not an expert on this, I thought I'd ask if this looks like a reasonable value, or would you advise going a bit higher or lower? FWIW, here are photos of attempts at -1.29 and -1.49, respectively, which to my eye both appear to be clearly bad (although perhaps if some gouging at the edges is expected, the -1.29 might be a better setting):





I've tried this with both MatterControl and IdeaMaker, and I get similar results with both of them.

 

I have no new photos, but cutting the extrusion rate back to about 85-90% does seem to clean it up a bit, so I may go with that and a slightly lower Z-axis value (M565 Z-1.35 or so). OTOH, I found a YouTube video with a test object that's supposed to show problems with over- and under-extrusion, and it seems to print OK with a 100% extrusion rate. Maybe at this point it's just fine-tuning beyond what I can judge. Thanks for the advice, in any event!