Oval circles at 45 degrees to bed

I’ve been using GRBL for some time and have become increasingly frustrated by my machine cutting ever so slightly oval circles that are at a 45 degree to the bed. We’re talking only by about 1mm from the top left to bottom right of the circle. So If I cut a 300mm circle with my Co2 machine, it will be 299mm across the 45 degree top left to bottom right and 301 from top right to bottom left. My suspicion for some time is that is was caused by backlash on the belts of my cheap extruded gantry. This was reinforced by the direction of cut which is set to clockwise in Lightburn. I had squared my machine using the 3/4/5 method successfully and even fitted some tensioning springs, but still suffered the issue. I finally bit the bullet and purchased a £360 Ruida controller with the additional license needed for Lightburn so that I could employ some backlash compensation. After a day of battling to setup the vendor configs on the new controller I attempted to dial in the backlash settings. Much to no avail and at the cost of utter frustration I am still getting this issue!!! My next question has to sit with the software. Am I missing something important here that I have missed or can anyone shine a light on why, after all the fiddling and cira 500 quid spaffed into the ocean I am still getting rotated ovals? I should also mention that, YES. my mirrors are aligned within a very small margin. This has also been checked across the cutting are to confirm if the Beam wonders around depending on where it is on the deck. Its very marginal which would be less than 10ths of a millimeter after its been through the 1" lens I used to test.
Welcoming the CNC veteran gurus who have bound to have run into this one in their old machine shop days before they retired into the world of hobby CNC lasers :smile:

Please try running the attached test for backlash;
Test.lbrn2 (73.5 KB)
and photograph the results.

Is your laser well calibrated for scale? X and Y line length at 300mm exactly the same?