Originally Posted by CliffordK

If you have the derailleur, go by what you have, not by what it says on a website.

You don't need a lot of clearance when you get to the largest sprocket, you just don't want the derailleur jockey wheels to be bumping on the sprocket.

B-Screw adjustment?

Anyway, by looking at the derailleur on the bike, you might be able to estimate what it would look like with a sprocket 2 teeth larger. Also look at your chain wrap (big/big vs small/small gear combos).

Did you say you were heading to the co-op? Perhaps they could loan you a 6 or 7 speed cassette to try out (or allow an exchange).