Print with whatever standard settings you use on your printer. Then check the X, Y, and Z dimensions for dimensional accuracy. Use the 10mm width of the section to verify your extrusion before adjusting the XY steps. 

Don't forget to check the diagonals for skew. IT should be 209.65mm (8.254 inch) for the square version. 

To find your calibration setting adjustment for each axis:

Using calipers, measure the inside and outside of the part printed along that axis (150mm & 130mm dimensions). Add the measurements and divide by 2 to get the average. This should be 140mm (5.518"). Divide 140mm by your actual measurement. If your number is greater than 1, the part is undersized and will need to be adjusted up. If it's less than 1, the part is oversize and needs to be adjusted down. Multiply that value by your current steps/mm setting in the printer. Repeat for each axis. 

ex.: I measure 150mm at 149.35mm and  and 130mm at 129.54mm

(149.35+129.54) / 2 = 139.445mm ← smaller than 140mm nominal

140mm / 139.445mm = 1.004 scale factor

Ender 3 uses 80 steps/mm for its nominal rate

Multiply 80 steps/mm x 1.004 = 80.3 steps/mm. ← Use this new value for your machine