How are you generating your gcode? Given that you have the same error in X and Y (although not sure how exact your measurements are!) it doesn't sound like a missing steps problem - although nothing can be ruled out at this point! But there can be issues with offsets and tool diameter settings in the CAM stage (gcode can be generated that allows for tool diameter at the gcode creation stage or makes no allowance which is done by the control software - Mach3 or whatever).

Can you post the gcode, assuming that it is something fairly short for just cutting a square?

I agree with Muzzer that you should be able to get very close to the correct "steps per" setting by calculation. On my own machine, this calculates as something like 400 steps per mm; accurate testing and measurement, at least as far as I could with my workshop facilities, meant changing this to something like 401 - presumably because of very slight errors in ballscrew pitch.