The little/little method will work with doubles or triples and gives the longest possible chain within the RD's wrap capacity. I've used the same chain length on doubles and triples, because the longer cage on the triple takes up the slack in the little/little position. There is no advantage to using a shorter chain. The only time the big/big method is a must, is when the largest cog being used is larger than recommend for the RD cage length. Then you better add length and know that the chain may hang loose in the little ring and several of the smaller cogs. It takes 4 teeth larger to use 1 inch of additional chain length.