You shouldn't need to lengthen the chain, since the big/big is the same and that length is not affected by the cage length. Of course, this assumes that you made the chain long enough to wrap the big/big (plus one inch) and just let the chain hang loose in the little ring and several of the smallest cogs. Otherewise, go back to the little/little method and you might get the suggestion to use a 1 inch longer chain.