Your firmware must assume that upper right is origin although you have it homing to lower left.
Try this:
$10=0 ; this changes your controller to report work coordinates instead of machine coordinates
G10 L2 P1 X-299 Y-299 ; sets a work offset to get you 0,0 for lower left
Is your actual working area 300,300? You may have an ease-off set that’s making it go to -299, -299 instead of -300,-300. You can adjust the specific values depending on how you want this to work.