Marlin 1.1.0 RC8 - how to manually correct the probed Z height?

So here is the situation:

I have a 250x320mm print bed, which is being probed with an inductive probe for auto bed leveling. The bed itself is perfectly flat (machined cast aluminum sheet)

No matter what leveling algorithm I use (3-Point, Linear, Bilinear) the result is always the same - perfectly level printbed with just one corner lifted. This one corner is always too high, no matter what I do. I cannot find a hardware issue with the printer (all screws tight, all carriages moving freely, no play, etc.)

So I have been thinking of some sort of workaround. Is it possible that I use the 3-point-leveling algorithm and somehow add 0,05mm to one specific probe point? This way I guess I could trick the firmware into thinking the sensor has probed this one point 0,05mm higher and thus is going to lift the nozzle additional 0,05mm there.

Here is an example with random values just to illustrate what I need to achieve:
Probe point 1: Probed z height at (x1, y1) = 2,1mm
Probe point 2: Probed z height at (x2, y2)= 2,0mm
Probe point 3: Probed z height at (x3, y3)= 2,15mm

So the firmware would calculate the tilt compensation using the three probed points. Is there any way to modify Probe point 3 to 2,155mm before the tilt compensation has been calculated?

An alternative workaround would be to correct the already calculated transformation matrix by manually making it always lift the nozzle additional 0,05mm at the problematic corner. This would be even better because it is going to work independent of the ABL algorithm...

I guess I should modify something in the ABL section of Marlin_Main.cpp but am not sure where and what....

Edited 1 time(s). Last edit at 04/16/2017 08:39AM by icefire.

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Self-sourced Mendelmax 2.0-based Reprap Machine -- Ramps 1.4 & Mega 2560 -- DRV8825 (Z@1A, [email protected], [email protected], E@1A) -- genuine E3D v6 direct setup -- 350W custom silicone heated bed -- ABS 1,75mm -- Marlin 1.1.0-RC7 -- Cura 15.04.6

Like this

G29 S3 Xn Yn Zn.nn ; Modify the Z height of a single point

Thank you but this is not quite what I needed. G29 S3 will overwrite the Z height of a single point. So it doesn't matter what the Z probe has measured. I want to make the marlin code always add a specific value to the measured height.

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Self-sourced Mendelmax 2.0-based Reprap Machine -- Ramps 1.4 & Mega 2560 -- DRV8825 (Z@1A, [email protected], [email protected], E@1A) -- genuine E3D v6 direct setup -- 350W custom silicone heated bed -- ABS 1,75mm -- Marlin 1.1.0-RC7 -- Cura 15.04.6

If you print surface is not flat... You need a Mesh based leveling system.

My recommendation is you bring up the UBL bed leveling system with a 10 x 10 mesh.

I suspect you won't have any reason to look at other bed leveling systems.