I know of no formula as such. As a base you would use the ampacity of the conductor X the number of conductors.
As with single conductors you need to take into account termination restrictions (110.14) and and derating factors.
The only added consideration in paralleling is if you share a conduit you must remember to apply the rules of 310.15(B)(2) (as you would on single runs when applicable)
You will probably end up with 500kcmil for 600 amps, but it depends on the caluclated load, derating factors, such as ambient, number of conductors (3 phase vs single; neutral or not) and if you are sharing one conduit.
There is no simple, one-step formula.
