Correct me if I'm wrong, but 6-600's would be
6 x 475 = 2850.
Derated for 4 conductors in conduit(480/277) at 80%
2850 x .8 = 2280.

Wouldn't it be 6x750kcmil?
6 x 535 = 3210
3210 x .8 = 2568