I ended up with a 3 gallon boil. Basically a gallon when steeping and added to two gallons of water after steeping. I would go by the target gravity on how much to add, but keep in mind this does have a strong fermentation that will need some headspace, so probably not much more than 5.5 gallons of beer in the fermenter.
My differences from the kits are standards people seem to follow. Steep grains in a gallon of water at 150-155 for 30 minutes (if you ever overheat this step up to or above 180, chunk it and start over, tannins lead to astringency in the beer) While steeping, heat 2 gallons of water to 165 and set the grains in the pot for 10 minutes, pull out the grains, dump in the steeped water and start the boil. After that heats to boiling, cut the heat and add the DME and stir it up, heat it back up to boiling and I add hops after the hot break when it stops foaming, and that is where to start the timing of the boil.
Started brewing February 2012. Still brewing.