Here in a nut shell is what I did. There are two things that need to be adjusted the best you can. one is the Bed must be parallel to the XY plane of the router. The other is that the Z axis is perpendicular the the Z axes plane. I fell back on the simple mode of Corner Probing. In 2.0.5.3 you can set it to probe the center too. That gives you two indicators of parallel error. I’ve tested it and measures quite accurately. I started by using my standard XYZ home using a touch plate and nail in the spindle. it step through the corners and center points manually. the first point (lower Left) is the reference point and saved in memory as 0.0. Pressing the control panel button moves the router to the (lower right) point and drops to Z0. Using the same touch probe, adjust the Z axes up or down until it just touches the button. Press the button again for it to move to remainder of the five points. There is a menu item that will display the the point values on the LCD or G29 T on a serial monitor like Pronterface etc. Now adjust the corner legs for that amount and repeat and repeat. It’s a very good way to measure non-parallelism. I made four small screw jacks to adjust the legs. If you turn auto-leveling on it will compensate all across the bed.
I hope this helps someone. The changes to configuration.h that hard. Let me know if someone needs help there. With auto-leveling on it will make up for minor board warping.
I’ve tested this only on MARLIN 2.0.5.3 so far, but it is worth the upgrade. I hope this ramblings are clear
Doug