I actually started doing just that before I trammed it.
-I leveled my support table.
-The Y-Axis’s are level and parrallel
-I leveled the x-axis (But did not check if it was right angle to the Y-axis) Would that be just using a framing square?
-Carriage and gantry are plumb
After I did all of that I had a noticeable difference in the tramming, it was about an 1/8" off over the 15" so I loosened the spindle mount and plumbed it to the wasteboard. Maybe by plumbing it to the wasteboard it made the issue worse? what would I plumb the spindle to if not the wasteboard?