Software power readings are not accurate. Do not trust it. 70-80W is actually 100-120W before the PSU, not even at the wall. It is very important to measure power with something, even cheap power meter plugs are better than software, and it is quite important to know the power consumption of your GPUs, specially if you are close to the PSU limit.

I agree with everything that philipma1957 said, and I would also put 6 GPUs per rig, specially with a 1200W PSU.