Determining Z probe offset x & y

I am unconvinced that I understand the correct manner to determine z probe offset x, and z probe offset y. I've seen multiple write-ups that seem to use entirely different coordinate origins, and the diagram in the Repetier Z Probe write-up shows the xy origin point centered between two extruders. I have a simple one extruder configuration, with an inductive probe mounted to the front and left of the extruder. My assumption had been that I merely needed to measure like this: