Calibration Object for both X and Y in increments of 5 mm, and the circles / cylinders are easier for measuring since there are no corners. On some of the printers I have used, the corners stick out a little bit more than the sides. The first 5mm height is useful to see how the printer works with the first layer being thicker, and then delta measurements for the 10mm and 15mm height show how accurate the printer is with the remaining layers. The width measures are good to see how accurate the X and Y are.