RDM's method is just about eliminating the twist in the bed - shimming adjusting the feet. Visualize an imaginary line that's the lathe spindle centerline projecting straight out down the bed towards the tailstock. Now imagine that the bed is a little twisted as though the tail end was unscrewed. As the carriage progresses down the bed towards the tail the tool is going to be pulled a little away from the centerline and just a teensy bit down. That's the quality that the RDM method is trying to measure and allow you to correct.
"In theory there’s no difference between theory and practice. In practice there’s a lot of difference.” Yogi Berra