Stephen,
What you have looks like it will work, although your A/C will always be on. I put a push button switch between the A/C thermostat and the fan speed switch so I can turn off the A/C.
Check if the white/black wire from the Mastercell has an inVert module installed to convert the +12V to ground. If it doesn't (mine didn't), you'll have to get an inVert module from Infinity Box or use a relay to convert to ground, otherwise you will fry the circuit in the Mastercell.