Bicycle Mechanics - New cassette - enough derailleur capacity?

I've decided to buy a second cassette so I can switch out when needed and have some lower climbing gears. Bike setup is:

complete 105 group
short cage rear derailleur
double crank 39/53
current cassette 12/23

I want to buy a 12-27 cassette, so was checking the specs of the drivetrain to make sure it fits.

First hurdle is that the Shimano website seems to have the specs for the two 105 derailleurs mixed up:
105 Rear Derailleur (For Double Crankset) - Silver - RD-5501-SS (http://bike.shimano.com/Road/105/componenttemplate.asp?partnumber=RD-5501-SS-S)
105 Rear Derailleur (For Triple Crankset) - Silver - RD-5501-GS] (http://bike.shimano.com/Road/105/componenttemplate.asp?partnumber=RD-5501-GS-S)

Specs from website (taken from the 105 triple derailleur page... which I assume is actually for double):
Max Sprocket = 27T - ok here - new max = 27
Min Sprocket = 11T - ok here - new min = 12
Front Difference = 14T - ok here - front range = 53-39 = 14
Total Capacity = 29T - ok here - total capacity = front range + rear range = 53-39 + 27-12 = 14 + 15 = 29

Have I calculated this correctly? It seems like I'll be on the limits of the specification, but everything will work fine???? I'll replace the chain at the same time as I'd rather wear each chain/cassette combo together plus I assume I'll probably need a slightly longer chain to cater for the extra cassette teeth.

sure it will work.. you may need as you know a longer chain as well you my need to adjust the B screw on the derailleur this is the screw that hits against the drop out of the bike (this isnt the limit screws). What that screw does is change the wrap of the chain on the rear cassette it needs proper wrap or you will experiance slipping of the chain.. real simple when you get going ... good luck

Der will handle the shift ok. Problem is chain might not
if it is correct for the 23t cassette, will be 2" short of
optimal for the 27t cassette. This would be a problem
ONLY for the 53x27 combo. All other combos would be
within the chain length as set up for the 23t. So if
you avoid the 27t, or if you add two links to split the
difference...... Actually it will probably shift into the
53tx27t but if you look at the R der it will be almost
close to straight diagonally, not good for it and a risk to
the bike. If your chain is already a link or two short of
ideal, then it probably won't even shift. Steve

Yes, it will work. And unless you cross-chain all the way (use the 39/12 and 53/27 combos), you're not really at the capacity limit. The "longest" usable combo is 53/24 and the "shortest" is 39/13 giving a required wrap-up capacity of (53+24)-(39+13) = 25.

Thanks! Good to see I had my logic sorted on this one.

I'm getting a new chain with the new cassette, but what you say is probably worth a try, Gonzo Bob. I'm pretty careful about cross chaining, so if it's unusable in the two extreme cross chain scenarios I'll not have a problem with the old chain (which is still low mileage).

rooftoptodd - thanks for the reminder on the B screw. I'll make sure to check it out.