You are correct, the load calcs are the key to this question. If the calculated load is 380 or less, the 500 kcmil is fine. If the load is over 380 but equal or less than 400, you will need 600 kcmil. If the load calc exceeds 380 amps, we really should be looking at a larger service. In this area the generally accepted wire for a 400 amp service is 500 kcmil.
If the engineer has specified 600 kcmil, that is what should be installed even if the load calc is is less that 380 amps. The engineer is permitted to exceed the code minimum.