i was dealing with this same problem too, The M557 command is using printer absolute cordinates without the tool offset, which is quite confusing. So if you want the points to be correct, you have to add/subtract the tool offsets to it. Kinda hard to explain so ill show mine
M557 X55:230 Y30:210 S40 ; define mesh grid
G10 P0 X-35 Y-13 Z-0.5 ;set tool 0 axis offsets
G31 X55 Y0 Z2 P25 ; sets z probe offests and sets activation value
so the nozzle is X -35 and the probe is X+60 from that. I want my first point to be around 20mm from the edge of the bed
so my first probe points is gonna be X55 as its X 60-35+20 = 55.
then just repeat for all axis and the end.