Originally Posted by Bobby Lex
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.
That's the answer. The Park Tool website will also show you how.
The derailluer cage length has nothing to do with it. The large / large combo determines the length. The difference between large/large and small/small determines what length cage you need.