Your grad school GPA does not figure into the LSAC GPA at all. Basically, you have to get a transcript submitted, but all they do is forward that on to the schools. How the schools then use it is anybody's guess.
If you did well, it's a nice soft factor to boost the overall quality of your app, but I wouldn't count on it to overcome a poor UGPA/LSAT and get you into a program you wouldn't have otherwise been admitted to anyway.
My grad GPA was excellent (3.9), but I don't really think it helped me any. My UGPA and LSAT were good enough to get me admitted where I am now without it. Had I gone for more of a reach school, I don't know if it would have made much of a difference or not. Probably depends on how big of a reach we are talking. It wouldn't overcome my scores to get me into Harvard or Yale...pretty sure about that.