https://github.com/PrusaOwners/prusaowners/wiki/Bed_Leveling_without_Wave_Springs
Following that tutorial and because I didn't want to update my firmware only to have relative numbers, I made a script to convert the output from Marlin to the relative numbers you need.
Script takes a file as parameter with G81 output, like that:
Recv: 0.16500 0.18463 0.19991 0.21083 0.21741 0.21963 0.21750
Recv: 0.16046 0.18181 0.20058 0.21676 0.23036 0.24138 0.24981
Recv: 0.15630 0.18067 0.20295 0.22315 0.24126 0.25727 0.27120
Recv: 0.15250 0.18120 0.20704 0.23000 0.25009 0.26731 0.28167
Recv: 0.14907 0.18342 0.21283 0.23731 0.25687 0.27150 0.28120
Recv: 0.14602 0.18730 0.22033 0.24509 0.26159 0.26984 0.26981
Recv: 0.14333 0.19287 0.22954 0.25333 0.26426 0.26231 0.24750
Then when you do:
$> ruby gen.rb output.txt
-0.065 -0.019 -0.013
-0.078 0 0.052
-0.087 0.023 0.017
You get the proper output relative to the center.
You can also access the converter directly online by going to: https://pcboy.github.io/g81_relative/
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