I think the SG you are looking for is 1.018.
Below that, Tannins are being extracted, from the hulls, during the sparge and will cause undesirable flavors in the finished beer.
Batch sparging is where you runoff the mash, add a new volume of water to the mash, rest to extract more sugars, and runoff again.
Typically, you would runoff all the water in to the kettle and then either boil or add DME to increase gravity, or dilute to lower, to desired level.
Another version is Parti-Gyle and this is where the first runoff is collected seperate from the second (and/or third) runoff to make different beer from the same mash.
An excellent source for brewery calculations is Designing Great Beers by Ray Daniels. It is very technical tho'.