Originally Posted by
kingfishr
If you need to lengthen the chain you would go by the large-large combination. If you need to shorten the chain you would of course consider smallest-smallest, and since most cyclists have multiple cassettes, you only consider if you are outside the range of what the derailleur will handle, and in this case it would be IF the chain is already on the border of being too long, and one link would put it over the edge.
That's incorrect. Chain length is always measured in the big/big. If it's too short you can destroy the rear derailleur shifting into the large sprocket. Not something I would roll the dice on. And I've never seen a chain length calculator that asks for the small cog size. I don't think the average cyclist has multiple sized cassettes that they're switching back and forth, but if so all they need to do is add or remove links during installation.