Summary

This test structure is supposed to help calibrating your printer.
It is a square ring with outer distance = 100mm and inner distance 90mm.

Instructions

Print the structure.
Repeat the procedure for both X and Y Axis
(The X axis has a marker included along its way.)

With a caliper measure the outer length and name it "A"
With a caliber measure the inner length and name it "B"

F = 190 /(A+B)
D = (100 B / 90 A) / 190

F is the Scale by which the axis is wrong
D is the Delta offset in mm,

In my case F is 1.008, D = -0.09
This means, I will multiply the steps/mm of my axis by 1.008 and
enter the new value.
As D is -0.09 mm this means, I will have to rasp the plastic item by 0.09 mm on all outer and inner faces to get correct dimensions.

Hope this helps!

Calibrating your printer