A regular 770 doesn't consume more than 230 watt's at most.
The reason they said 5-600 minimum is partly because they have to guard against bogus advertised power supplies. ( potentially including your own )But also because margin doesn't hurt.
The graphics card and CPU, along with most other components take their juice from the 12 volt rail.Some power supplies have a total delivery of 600 watt simultaneously, but only 400 of those are from the 12 volt.
Others actually deliver the full load and then some on the 12 volt rail.
Check your PSU sticker for exactly how much your power-supply claims that it can deliver on that rail.
Your math is sound, but even if you have 450 on the 12volt, I'd probably feel that it 100 watt is borderline too close for comfort.