From Sheldon:
The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that they could be connected (outer link meets inner link), then make the chain one complete link (one inch) longer than that. In almost all cases, this will give the optimum length.
http://sheldonbrown.com/derailer-adjustment.html#chain