Be sure the extruder is calibrated using the thinnest layers you intend to print before adjusting the X and Y axis steps/mm.
The mathematical calculations based on motor steps per rev and pulley tooth counts are first order approximations. Cheap pulleys cannot be expected to have accurate/precise diameters and therefore you may have to make adjustment to compensate. A small error in the diameter (number of teeth notwithstanding) is multiplied by pi in the linear term. If the extruder is calibrated, yes, you will reduce the steps/mm in X and Y to obtain the accuracy you desire.
You might consider printing a larger object before adjusting the calibration. With a 30mm object, the error in your measurement is likely to be a significant portion of the overall error. Printing and measuring a larger object may allow you to achieve a more accurate result.
I use this cube for calibration and for checking the alignment of the axes:
https://www.youmagine.com/designs/alignment-and-calibration-cube.
My disdain for dual Z axis motors stems from the time, material, and effort wasted in printing this cube and making required adjustments whenever the X axis becomes misaligned relative to the Y and Z axes due to failure of the motors to remain in sync, or upon a more mundane occurrence, such as a child turning one of the Z axis screws with their unaided fingers.
