Knocked up a quick small application to create Gcode for levelling your bed.
More features to be added yet.
Please note, that the program takes it for granted that you are using the top of the material as Zero.
So when setting the depth to say 3mm you are in fact telling it to clear 3mm down from zero point (-3.0mm).
Please let me know if any bugs in the calculations. But seems okay this end.
Sorry only Metric at the moment, will add Imperial later.
Cheers
Sean.