I test both big/big and small/small and pick whichever method indicates a longer length.

Sram derailleurs by their design (single sprung pivot) are insensitive to chain length as long as it's long enough, but work with a smaller range of cassettes. Campy derailleurs work with a bigger range of cassettes, correspondingly depend more on chain tension to keep the top wheel at the right distance. Shimano are in between.