G28        ; home
M851       ; display current setting
M851 Z0    ; set offset to zero
G28        ; home
G1 Z0      ; move the nozzle to height 0
M851 Z-1.7 ; the new value 
           ; increase if the first layer is too sick, 1.7 for example
           ; decrease if the first layer is too thin/the nozzle scratch or hit the bed, 1.5 for example
M500       ; store the new configuration