I first would make sure that the belts are tensioned, the wheels are tight, but not too tight, and that the pulleys are not loose on the motors. Then I’d put a straight bit in the router so you have a sharp edge to gauge the measurement. You need to keep doing the math until the movements are as accurate as you can get it. It’s a ratio of what you want to what you get. (20/19.875) x $159.70 = 1.006 x 159.70= $160.70 as the new number for the Y axis. These numbers could change after you tighten things up though…so it’s just an example.