Bicycle Mechanics - Compact Cranks and chain length

On my current bike, I've already perfected the chain length using the method Sheldon Brown recommends. This is on a 10 speed Ultegra double 53x39 setup, with a 12-25 cassette.

Now I just ordered an R700 compact crank (50x34), and a 12-27 cassette to help me ride up some serious mountains on an upcoming vacation.

So here's the question - I am going down 3 teeth in the front chainring, but adding two teeth to the largest rear cog, for a net difference of +1 tooth (I assume I'm correct on that part). Will I need to add a link to my chain to accomodate the new setup? I ordered a new Ultegra chain just in case, but I'm trying to plan the install in my head before the parts arrive.

Other than that I'm good on the install. Here's one weird thing though, the Ultegra chain that came on my bike actually has a SRAM style master link from the factory (LBS says it came that way, they didn't add it), which is nice. Hopefully the new chain I ordered will have one, making my standard to compact conversion an easy process whenever I'm lucky enough to leave the flatlands of Florida.

I debated on whether I needed to drop money on a new cassette to get the 27 cog, but figured it couldn't hurt me in the mountains, since it still has a low cog of 12 for those wicked descents.

Actually you are -1 tooth, or in the 1/2 link set range.
IF your current chain has 1-1/2 link sets "slack", you would theoretically remove 1 link set with your new combo. I probably wouldn't bother unless I was going to swap the 12-25 back on, while keeping the compact rings.

You may need the new chain anyway. If the current cassette and chain have been used for a while, say 1500 miles or more) they probably have worn together and the old chain will skip on a new cassette.

If both are relatively new, you can probably use the chain length you have. Adding 1 tooth requires only adding 1/2 of a "half-link" so, unless your chain is very marginal or too short in big-big with the current ring and cassette, you should be good.

Bill you're right, I meant to say -1 link. The bike has about 900 miles on it, and the chain has been kept clean, so I'm thinking chain stretch shouldn't be an issue. Good point though, I'll throw the Park chain checker on there before I make the swap.

Thanks.

Bill you're right, I meant to say -1 link. The bike has about 900 miles on it, and the chain has been kept clean, so I'm thinking chain stretch shouldn't be an issue. Good point though, I'll throw the Park chain checker on there before I make the swap.

Chains are cheaper than cassettes. I don't necessarily feel the need to change the cassette every time that I install a new chain but I always install a new chain if I replace the cassette.

good point, I'll just keep the new chain/cassette combo for use with the compact, and my original chain/cassette for use with the standard. After a few practice runs I anticipate the ability to swap the drivetrain out in just a few more seconds than a front wheel change in the TDF.

Hmm, that makes me think, maybe I'll just carry the parts with me and swap for the tough climbs. Nevermind, that'll probably lead to grease on my white shorts and bar tape.