SRAM chain length is the same as any other chain length. Assuming that you have a ring spread and cassette spread appropriate for your component choices, the selection of the best chain length is pretty simple.
First, set the 'B' screw to fit your cassette. Selecting the smallest sproket and the small chainring. You want to choose the chain length that gives you enough tension to allow the chain to clear the jockey pulley. It is a bit of a fine line between choosing a 'shorter' or a 'longer' chain, as the spread of links sometimes forces a choice between the two. Generally, my recommendation is based on the use to which the bike is to be put.
For an all-purpose bike (or an inexperienced rider), I usually recommend erring on the 'long' side. This is how I set-up my wife's bikes and my girlfriend's bikes. A little bit of chain slap and play may be ok in that application. Also, it may allow the rider to choose a larger cassette for use in the mountains.
For a race bike, the better choice is to err on the 'short' side. The additional tension on the chain may save a dropped chain or a chain-suck problem one of those desperation cross-chain counter-attack 'incidents' that seem to always happen on a bumpy, off-camber change in terrain (hmmmmm. Why does that always seem to happen?
) The 'risk' to the derraileur is probably acceptable in a pure racing application where you have no-one to blame but yourself for cross-chaining and throwing 1,200 watts through it (whereas the wife/GF might kill you).