In beaurocratic limbo
Join Date: Mar 2006
Location: Los Angeles, CA
Bikes: Specialized Allez, K2 Razorback
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Yes, there is a margin of error so a 30 will probably be fine. If it doesn't work, you'll get some rubbing between the 30t cog and the derailleur pulley.
Now, make sure you're not too far past the maximum chain wrap capacity as well. If you go too far with that, it can yank your derailleur apart (happened to me) if you cross chain in the big/big combo.