The build area is not a circle and doesn't resemble a circle. This is NOT the entire build area it leaves 1-2 CM on the outside. This object is much better at testing your X/Y/Z tower endstop calibration. Since it uses most of the printable area, small offsets will show up in the print. To find the true area just take the outer shape, expand it out by 25mm and print it with concentric outer shells inside to outside. Once the arms smack the towers you'll have a good estimate to go on.