Welcome to the SeeMeCNC Community

Ok, so I think I have my machine dialed in pretty well, but I have a few issues that after a few weeks of reading I'm not able to pinpoint which settings I need to tweak to get things as perfect as I'd like.

First, here's the 20x20x10mm calibration cube.

Bottom layer (note first layer is bulging a bit).
[img]http://i.imgur.com/wVihEHx.jpg[/img]

The first layer thickness is 0.25, which is what I set it to. Printed at 40mm/s infil, 30mm/s outside, and 30s minimum per layer.

Here is a view of the top and along the X-Z and X-Y axis:
[img]http://i.imgur.com/L4moxgQ.jpg[/img]

And along the Y-Z axis, also looking at the top:
[img]http://i.imgur.com/DjCS3t0.jpg[/img]

The material is PETG eSun Blue, measured filament thickness is 1.74mm nominally, extrusion width is 0.3mm on a 0.35 nozzle, E-steps/mm is 101.875.

Measurement across X-Y axis:
bottom: 20.43
middle: 20.15
top: 20.24

Measurement across Y-Z axis:
bottom: 20.28
middle: 20.05
top: 19.97

Measurement bottom to top:
Front face: 9.99
middle: 9.95
back face: 10.05

Any thoughts or criticisms are welcome. What should I be looking at to fix the bottom layer? I set the Z height so it just starts dragging a piece of paper. I think I have each axis stop screw set to pretty good values. Horizontal radius is 130.750.

What setting(s) causes the X-Y axis to be slightly larger than the X-Z? Why does the part seem to bow inwards in the middle layers? Tension/shrinkage due to the infill?

What should I be looking at next, or am I being too retentive?

Thanks in advance.

I think the best thing to do to tune your printer to the best it can be is to download calibration prints like this one:
http://www.thingiverse.com/thing:5573/#files
These types of prints can help to diagnose what needs to be adjusted if anything. Your calibration seems very close
to being on target with just a little more tweaking to be perfect.