Print Not Correct Size

[kwixson]

My Rostock MAX prints are dead-on exactly now long they should be, but not quite the right width. A 20x20mm cube, for example, is 20.01 long, but about 19.65 - 19.75mm wide. It's pretty consistent (the variability I attribute to my measuring) even when i change the print. A 50mm wide print, for example, is also short by about .3mm. So, what do I do to troubleshoot this? I'm used to my Prusa, which is affected by belt tension and settings of e steps in the firmware, so I adjusted the belt tension on the RoMax, but obviously wouldn't want to touch the e steps.

If it matters I'm running a 1 week old Rostock Max from a Mac using Repitier-host and the Marlin firmware from John Oly's git.

[Polygonhell]

My Rostock MAX prints are dead-on exactly now long they should be, but not quite the right width. A 20x20mm cube, for example, is 20.01 long, but about 19.65 - 19.75mm wide. It's pretty consistent (the variability I attribute to my measuring) even when i change the print. A 50mm wide print, for example, is also short by about .3mm. So, what do I do to troubleshoot this? I'm used to my Prusa, which is affected by belt tension and settings of e steps in the firmware, so I adjusted the belt tension on the RoMax, but obviously wouldn't want to touch the e steps.

If it matters I'm running a 1 week old Rostock Max from a Mac using Repitier-host and the Marlin firmware from John Oly's git.

There isn't much you can do in software to correct out of square. All the math is based on all 3 axis being identical, you can change the overall scale by adjusting the arm length in the firmware, you can't do it separately for X and Y.
On the mechanical side it's about making sure things are symmetric.