The way I do it is to use HWiNFO64 to monitor voltage, and set core voltage to 900mV. If that takes under load (Cast/Stak is stable, and HWiNFO reports core at 900), then I'll try dropping it to 881.

Of course, I've only been successful in about half of my cards (7 of 14), but many of my others are in the low 900s, which will push the draw up to 180W or so which isn't bad; I could probably back that off and hash around 1900 at 150W, which may be more palatable for some.