With 400A of 208v, you can run 80 S9 13.5, should be able to load panel with 40 breakers, than on a few of them just run 1 S9 and 2 L3. Obviously just watch the panel when loading up, transformer with poor harmonics will tend to draw lower voltage increasing amperage.
80 s9 running at 6.5-7 amps each would be well over 500 amps. How exactly did you make your math?
400 amp panel @ 80% capacity is 320 amps of usable power. S9s drawing 7 amps each would mean you could run 45 total. You would probably want to run less because you need to leave power for ventilation and other things.
I also run 3 phase 208. My setup is like this.
I run 3 phase all the way to the PDU. To balance power I run them as follows....
Miner layout per PDU:
Phase A Phase B Phase C
PDU 1: 2 1 1
PDU 2: 1 2 1
PDU 3: 1 1 2
I run 9 breakers per sub panel and balance the 3 phases to the panel this way leaving the 10th breaker for ventilation. The draw across the 3 phases as measured at the subpanel stays within a few amps of each other. (Not all machines pull the exact same amount of power, I have some S9s pulling 1380w, some pulling nearly 1500w depending on batch and PSU used.)
Not sure how your pdu applies one or two miners on one leg only, should be swazzeling across the legs, maybe there are different types i dont know as we have not used them. When we run 2 pull breakers on 3 phase
13 miners = 84.5amp on leg A and B
13 miners = 84.5amp on leg A and C
13 miners = 84.5amp on leg B and C
ending current per leg is about 145 amp, 52 kva. within 80% for 75kva transformer feeding the panel.