See this graphic (by courtesy of Voron)
You would set:
position_endstop: 255
position_max: 255
This allows your printer to reach your desired X=0 and Y=0.
In your slicer’s configuration you would set a bedsize of only 250 x 250 making sure that your print actually ends up on the bed.
In your case X=0 and Y=0 is the start of your PEI and the size of your PEI is the bedsize for the slicer.