The machine needs to be oriented like this:
Make sure that Origin is set to lower-left in Device Settings:
Also, your dimensions are actually 255 mm on X (wide) and 420 mm on Y (tall). Not 420x255 mm. Make sure that’s what you put in Device Settings for work area.
Also, for some reason your Y max travel is set for 440 mm. Let’s change that to meet spec.
Run this in Console:
$131=420
If you find that you have room to spare then you can extend this later.
After homing, your the position of your laser if you do a “Get Position” in the move window should be 0,420.
Your laser does not home to 0,0. And lower-left is your origin. Do not get confused by people saying that home and origin is the same. It’s not in your case.