Multiple +12V rails are splits from a single +12V to provide safety in OCP. This prevents a single rail from carrying so much current that it gets too hot. But to calculate, you would still need to know the total current for the +12V rail. In your case, there's a footnote in the PSU that says that the combined +12V cannot exceed 384W. In other words, your PSU has a total current rating of 32A on the +12V rail. That's more than sufficient to run your HD 7790.