by MLworx, published
My printed parts were always about 0.9% too small - to find out if there's lash in my belts or a wrong value in the firmware (64 steps at X and Y before editing) I printed this L.
Measure the 140mm in X and Y and do the same for the 20mm.
If your part has the same deviation (in %) your firmware value maybe wrong, if you have different deviations you maybe have too loose belts.
My firmware setting is now 64.64 (+1%) for X and Y.