How to use "Bed Level Correct" ?  

How to use "Bed Level Correct" ?

Firmware 3.0.6 includes the "Bed Level Correct" setting under Calibration. Four values can be set - left, right, front, rear. Not much info in the manual about using them. Yeah, "advanced feature", so left out of the manual !

Is anybody using this ? Any info on this will be much appreciated.

Before continuing, let me say that my printer is almost well built. XYZ calibration says everything is good. I am looking to tune this further by fixing the bed level.

One logical point to begin this tuning seemed to the output of the Z calibration. I connected the USB cable. Ran Z calibration thrice. Got this on the serial. Bed point 0,0 is considered zero (reference), and the values are w.r.t that:

echo:enqueing "M45 Z"
Bed point 1,0, differences: written -0.00250, read: 0.00000
Bed point 2,0, differences: written 0.22833, read: 0.23000
Bed point 0,1, differences: written -0.18833, read: -0.19000
Bed point 1,1, differences: written -0.30250, read: -0.30000
Bed point 2,1, differences: written -0.20167, read: -0.20000
Bed point 0,2, differences: written -0.61333, read: -0.61000
Bed point 1,2, differences: written -0.95000, read: -0.95000
Bed point 2,2, differences: written -0.89000, read: -0.89000
echo:enqueing "M45 Z"
Bed point 1,0, differences: written -0.00750, read: -0.01000
Bed point 2,0, differences: written 0.20750, read: 0.21000
Bed point 0,1, differences: written -0.20333, read: -0.20000
Bed point 1,1, differences: written -0.32250, read: -0.32000
Bed point 2,1, differences: written -0.22083, read: -0.22000
Bed point 0,2, differences: written -0.63333, read: -0.63000
Bed point 1,2, differences: written -0.96833, read: -0.97000
Bed point 2,2, differences: written -0.91583, read: -0.92000
echo:enqueing "M45 Z"
Bed point 1,0, differences: written -0.05667, read: -0.06000
Bed point 2,0, differences: written 0.11250, read: 0.11000
Bed point 0,1, differences: written -0.17833, read: -0.18000
Bed point 1,1, differences: written -0.35417, read: -0.35000
Bed point 2,1, differences: written -0.30583, read: -0.31000
Bed point 0,2, differences: written -0.60417, read: -0.60000
Bed point 1,2, differences: written -0.98417, read: -0.98000
Bed point 2,2, differences: written -0.99667, read: -1.00000


There is a run to run variation (expected with low cost sensors). However, there is a pattern in the values that hints at the levelling of the bed. The rough simplification of the above values yields


Bed Point (0-2,2) -0.6 -0.9 -0.9
Bed Point (0-2,1) -0.2 -0.3 -0.3
Bed Point (0-2,0) +0.0 +0.0+ 0.2


Clearly, the back of the bed is lower than the front.

I am unable to infer anything else at the moment. Two questions at the top of my mind:
1. Is it possible to derive the bed level values from these values..
2. If yes, how ?

Thanks for any input!
-- Shree