Originally Posted by
FBinNY
Shift to small/small, and wind the RD cage clockwise. If you can do so enough to pull up the slack, then the chain is short enough. If you can't pull out the slack before hitting the RD's cage stop, then it's a bit long.
In any case, shortening the chain carries it's own risk, which is far more serious than a slight long chain. Most mechanics measure chains by the big/big+1 method, which yields the shortest safe length. So, shift to big/big and check how much chain you have to spare before cutting.
If you are within specs for the capacity of the rear derailleur with the 30 tooth cassette, then the small-small method above will allow you to safely change back if you find you want to. If you do big-big with the 28 and then go back to the 30, you risk putting your derailleur into the spokes if you shift into big-big.
The chain should not droop in small-small, unless you are exceeding the specs for wrap. That may be a reasonable thing to do, but you would be well advised not to pedal in combinations like that, as it can skip (IME).