created this to check bed calibration. model walls are 1 mm wide model is 1 mm tall 280 mm dia. outside (white line on Onyx bed) small arrow in center points to front of machine. drawn in Fusion 360

Print 1st layer .4 rest at .2 print let bed cool carefully remove measure thickness at different points to see how flat your machine is printing.

Attribution - Creative Commons

Calibration, RostockMAX, TrickLaser, HE280, Brian, TestPrint, Fusion360