@Woodcraft Problem solved, thank you so much for the article,
I did a couple of things:
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Rechecked the belt tightness and found that when compared to the maintenance video my belts were too tight.
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There was a bit of movement in the X axis frame, the M4 screws used to mount the X axis to Y axis are countersink type and over time the hole on the metallic frame has probably got slightly enlarged and it was not holding it firmly even though the screw was fully tight. After replacing it with a Pan Head type screw there was no more play.
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After all this, the cutout reading stabilised at (-0.25mm). I then used Kerf Offset option in layer cut setting to adjust rather than calibrate axis.
I have tried different cutouts dimensions ranging from 20 mm to 190 mm and the results are accurate and consistent
Thank you 