Since you are already running miners on the circuit, you can just get a clamp-on ammeter to measure each leg at the 150A breaker. The limit is 120A. If any two legs are less than 113A you can add one miner between those legs.  

People are taking liberties with the specs. 13.5 TH/s S9 spec is 1323W +10% at 25C ambient assuming the PSU runs at 93% efficiency. These specs are Bitmain's calculation model, not done by the mean of statistically significant sample measurements. Please also consider the power increase required to operate in 40C ambient conditions for both the miner and the PSU.

Note their power number is power with tolerance of +10%. It does not say + or - 10%. The 0.098 J/GH is 10% lower than the mean. That's why they stipulate to "expect" +10%. They really wanted to be under 0.1 J/GH because that sells more miners.

When I read all this I do not expect to measure 1455W maximum per miner. And I don't. I measure 1530W mean, at 25C. I don't know how much power to expect at 40C since I haven't been there to measure. I am certain it'll be higher.

All that above to share a point. The 80% circuit limitation is there partly to cover for temperature and specification maximums. Because Bitmain chewed up 10% and didn't account for 15C over normal temps, the circuit is actually at its practical limit if calcs show 80%. All that said it's no big deal in my mind as the breaker will just trip if you overload it. That is, assuming you follow all the other rules.