I stuck my indicator back in the collet and took 15 readings relative to 0,0 in a 3x5 grid.
What I see is a slight bump in the middle of the wasteboard running from front to back or Y direction. Worst case along the Y axis is 0.73mm over 800mm and that’s in the center of the board from x610, y0 to x610, y800. The others taper off from this ridge toward X0 and X1210. But generally I see the back (Y800) is slightly higher (about .51mm) than the front (Y0).
Left to right along X it is almost dead flat measuring at Y0, Y400, and Y800. Worst case is .22mm difference over 1210mm (0.009" over 48").
I might look at loosening the mounting bolts for the QCW rails along the back side and see if they will drop down just a smidge. Maybe just loosen and retighten and see if that helps. I don’t see it getting worse. If I can flatten it out a bit more before I take the surfacing bit to it that would be good.