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You shouldn't calibrate the steps/mm setting for any axis other than the extruder.
Unless, of course, you have a very large printer (300mm and above) or are using something like spectra line instead of timing belts. In that case, print the largest scale test that fits on your print bed, oriented in the direction you want to calibrate. Keep in mind that the test parts are slightly larger than their nominal size.
Then measure the distance between the holes in each end, preferably going from the 100mm spot on your tape measure upwards. Or use any other precision measuring device that you feel comfortable with. If you can match up the distance between the holes by eye to the measuring tape (or other measuring device), you're close enough. FFF 3D printers are not precision machines, so the error of matching is negligible compared to the base accuracy of the printer itself. Measuring holes instead of outer dimensions ensures that a (slightly) miscalibrated extruder or other errors won't affect the final measurement.
You can calculate your new steps/mm like this:
[old steps per mm]*[nominal distance]/[measured distance] = [new steps per mm]