No comment form Snapmaker and I am now sure I am correct.
Put two small 5mm blocks on G1029 levelling points i1 j2 and i3 j2. Do a touch screen level. Then get the head to move from X0 to X320 along the line (Y160). The head moves up and down but the high and low points correspond with the leveling mesh points given by G42, not those actually probed in the Snapmaker leveling process. which are 19mm to the right. If you try this make sure the head is up slightly Z3 or it will crash into the bed on the right side.
As this is something wrong with the firmware I have raised it as an issue on GitHub - not response yet.
If you want to to a bed level at the same points that are used for leveling when you print you can turn off Workspace, level using G1029 and turn workspace back on.
I use the macro detailed below. You need to put a number in the “probe Z offset goes here”.
Calculate this by doing a touch screen calibration and when you do the manual bit where you bring the nozzle down to the levelling sheet make sure you record how far you bring it down and this is the “probe Z offset number”. This can be adjusted later very easily just dont make the number too big as the script moved the nozzle down to the bed.
So run the macro from something USB connected to the printer ( Luban or OctoPrint with Macro plugin) Once done print something small to check level. You can adjust the level up or down slightly using G1029 D x.xx If x.xx is a negative all printing will move towards the bed, move away if positive. No real idea why but I seem to need to move it all down 0.01mm with G1029 D -0.01. Change the M140/M190 values to change the temperature at which the bed is leveled or comment out to do it without heat.
;bed level script bed heated to 65c
;bed heat is left on as usually doing a print after this.
G21 ;set units to millimetre
G90 ;set absolute positioning
M425 X0.02 Y0.02 Z0.02 F1 S0 ; set backlash to 0.02mm
M140 S65 ; set bed temp for bed level, heat while homeing
G28 ;do this now, done in G1029 but takes ages
M190 S65 ; wait for bed temp
M206 x0 y0 ;remove worspace offset prior to leveling
M500 ;save change
;Auto Level
G1029 A ;start levelling
G91; relative positioning for Z offset move
G0 X0.5 Y-10 ;Adjust for difference in probe offset
;*****************
G0 Z-x.xx ; Device specific Z offset, reduce to bring print closer to the bed (bigger minus number). (equivalent to M851)
;*****************
G1029 S ;save data
G1029 D0 ;end levelling
G90; restore absolute positioning
M206 x-19 y-10 ;Put workspace offset back on
M500 ;save change
G28; home all for safety