To supply power "according to the book" you should think of the slot as good for 75 watts, and each six-pin aux cable as good for 75 watts, for a total of 225 without doing anything heroic.
In theory, an 8-pin cable is good for 150 Watts, but that is because its wiring is heavier and the power supply is ready willing and able to send that much power to it.
Just splitting the 8-pin wiring and powering it with a six-pin cable is not a recipe for guaranteed success. You MAY get away with it, but if not, your graphics card will overdraw the power supply, causing it to suddenly power down under load.
The "approved method" for this case is to use a different source, such as a SATA connector and a stunt cable to pick up more power. If that does not work, an external power supply should be used to power the card.