F360 is doing my head in ;)

I have my front face set up with my chosen work coordinates system (WCS) as front left corner, Z is of course up/down.

Now, i presumed that if i wanted to machine a feature on the bottom or underside face of that part i could rotate it round, do a new setup and set my WCS as top left corner with Z again now being upwards???

I have spent an hour messing and given up for now - it will not let me position the WCS how i want it and persists in putting either the x or y axis in the wrong direction, i post processed a trial and it does not work as it is off the table in Mach3.

Any ideas how to machine stuff on the underside of a part???