It doesnt matter where you draw it in CAD you just put it in the same place in Mach3
not sure if we have our wires crossed phil ?
in CAM not "CAD" you dictate atributes of the tool path, if your tool path starts in the center of a block you need to do a bit of maths phyisicaly place the tool in the center of the block to start

if your starting from a corner you only have to zero in on that corner set your hight and the gcode will jog to the centre and start cutting