Check out the John Bridge forum for lots of discussion about SLC. I used it in a couple of places and was very happy with the results. For a large area like yours, you might want to rent a mixer or have several people with 1/2" drills, paddles, and 5-gallon buckets working steadily. Things I learned or wish I'd learned during the process:
1) The cheapest SLC you can buy at HD is just as good as the expensive stuff. Shop around. I used LevelQuik or some such; wished it had been called LevelNotQuiteSoQuik.
2) You can mix one bag in a 5-gallon bucket. You can't mix one or 2 bags in a rented mixer -- it doesn't mix thoroughly enough. You probably could mix 4 or 5 bags in a mixer, but I didn't try -- I went directly to the buckets after failing to mix 2 bags nicely. Buckets are also easier to manage for pouring over a large area.
3) Don't mess with it. Pour it and let it do its thing, although it may need some help running up to a knife edge (not your problem). Over a large area, pour it in lots of piles and let the piles merge. I'm not sure how large an area it will adapt to on its own, but it's probably on the order of a 6' diameter circle.
4) It's about the consistency of pancake batter, but runs like water. If you set a form board up at the edge of your patio, for instance, the SLC will run right through the smallest gap between the form and old concrete. After you set your forms, run a bead of cheap caulk along the gap to seal it.
5) As Jim said, follow the directions. If the directions say 6 quarts of water per bag, that's what you use.
6) Also as Jim said, temperature matters. I had a bunch of gallon jugs of water frozen beforehand to use to cool the mixing water to extend the working time.
You won't believe it until you see it work. Very neat stuff.