Truing the X/Y travel to maintain 90 degrees.

I inherited this MPCNC with Mini-Rambo and Marlin V1.3a recently. I rebuilt the support legs to raise the rails by 3". Effective max travel is about 750mm in the X direction and 430 in the Y. At the max travel extents, the X/Y describes a 92 degree angle. How do I true it back to 90?

Thanks for any help.