Creating an OATS (Open Area Test Site) for Radiated measurements (pre-CE)
Sounds good, but

Remember a random location above real or metallic ground does not give clean measurements because of reflections from the ground.

So for CE/FCC/CISPR measurements, you need to raise and lower the receiving antenna to find the strongest signal. Also rotate the DUT and its wires all around. Of course you need to do this at each frequency, but in practice you just hunt down the strongest position for the few strongest spurs.
Without rotation and searching over height, your results will be dominated by random changes in the gain of the transmit antenna system (DUT and its feed wires) and reflections causing cancellation at different frequencies.

Finally, if you have a calibrated spectrum analyser and reasonably well known antenna, you don't need to calibrate with a known source. You can simply calculate the transmitted power from the measured field strength. It would be a good check of your method though.