Photo Credit: Courtesy of 3DStockPhoto ( image)
G29 determines the shape of the bed by probing the bed. This will set the shape of the bed with respect to the sensor trigger point. The z-offset is needed to set the offset between the bed itself and the nozzle with respect to the sensor trigger point.
The sequence to determine the offset is:
M851 Z0; // Set the Z offset to zero height G28; // Home Z in the middle of the bed G1 Z0; // This will move the head to zero height; M211 S0; // This will disable the end stops so that you // will be able to proceed lower than Z=0
Now adjust Z height to fit a piece of paper and note the negative Z height (either through the LCD or through an application over USB)
M851 Z-1.23; // Define the Z offset M500; // Store the settings M211 S1; // Enable the end stops again
Please note that -1.23 is a fictive value that should be replaced by your own value.