Originally Posted by
Lazyass
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.
Strange because even the link you provided says to check small-small:
"Shift bike to smallest chainring in front and smallest cog in back. Inspect the section of chain between lower derailleur pulley wheel and bottom of smallest chainring. There should not be an obvious sag in the chain. Check also that the lower section of chain does not rub chain at upper pulley. It is normal for there be low chain tension in this position, but the chain should not sag. Sagging in this position indicates a chain that is too long."
in the OP's question he is going from a 30 to a 28 tooth max so there is no risk that the chain is going to be too short.