Andy: Do the calibration before every ride. Your tire pressure will be different. Temperature and humidity will be different. You might weigh more or less or have on something heavy or just left your keys to the vault in your jersey. :), Yes, it's a pain. But you do want valuable data, right?
Have you checked out the demo of the new cycling peaks software at www.cyclingpeakssoftware.com
? Download it and import your CT data. Fun, fun, fun.
"How wonderful it is that nobody need wait a single moment before starting to improve the world." ~Anne Frank