Gerry is correct the 0,0 point of origin is just that it does not mean the starting point of the Gcode sequence. NOW some CAMs do use 0,0 and some don't. Most of the time YOU have the option as to WHERE you want the code to start.
I can say that doing it that way is going to be hard to follow Gcode wise as you are working in negative numbers. IF you set the POO as the lower left then everything is in positive numbers.
Just a thought, (;-) TP