skewed prints: scaling or offset (or both)

I have a hollow test square 100mm wide with 5mm thick walls (2mm high)
My machine is pretty straight, metal corners, best I can do mechanically.
I have duet ethernet and run the autocal ok, I also measured the H parameters.
My overall dimensions are 101.7 (should be 100), but the 5mm wide walls measure 5.4.
To get something like this would require both a scale and an offset error (y=mx+b of course)
Regardless, how do I adjust this scale if not handled by the autocal routine?
Even more, where is so much offset coming from and how do I fix that?
thanks

The scale of your final printed part depends on at least three things:

1. How accurately the effector travels the requested amount. This may be non-linear if your configured tower steps/mm or diagonal rod length is incorrect. So print a pattern of thin grid lines to check scaling and linearity. If it is linear but the scaling is wrong, you can use M579 to correct it.

2. How much the plastic shrinks after the print cools.

3. How much over or under extrusion you have, which will make the perimeters thicker or thinner than they should be.

The measurements you report suggest that you have either non linear movement or over extrusion.

HTH David

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks David. Do you know how to easily create a one-path pattern? I attempt to make a wire-frame with wire widths just larger than the extrusion width HOPING the slicer will fit just one path everywhere but that is really clunky and I'm not able to make this work today at all.
Or am stuck doing the entire g-code file by hand? That will require a bunch of trial and error
This would be a really nice utility ie take a vector path say from Inkscape, with control over extrusion width and speed.