I guess for X and Y you could split the difference and use M557 X5:285 Y50:290 S20 instead. That's a bit more optimised. Gets you an extra row of points in the Y direction and centers the X points a bit better.
@psyfor
while printing use babystepping to make the final adjustment for a perfect first layer, then apply the final value for babystepping to the Z offset.
I think you have the 3-pin connector reversed in the PWM connector. AFAIR the brown wire is ground, which is the rightmost pin on the DueX PWM connector. That would explain why deploy/retract isn't working.