you really don't HAVE to do anything, as the powertap will re-zero as soon as you coast for about 5 seconds.

But if you don't expect to be doing any coasting or want to super accurate even at the start of your ride (like before a triathlon) these should be the steps to re-zero the powertap:

https://support.garmin.com/...t/case.faces?caseId={5954e2f0-9dbd-11e0-d01c-000000000000}

Garmin calls it calibration, but it isn't really calibration, it is just zeroing.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

I am new to PT too and had similar questions. I have the 910xt and as soon as I turn it on and put it in bike mode it displays a message something like, "power meter detected, calibrate Y/N". I select yes (The bike is stationary, ie, not biking, pedaling or even sitting on the bike). It concludes the calibration and displays a value of around 510 ( which I understand is okay, ie less then 500 is bad) and the torque readings are 0. That't it, I then ride and assume that my power readings are fine.

That's nice, but the poster has a 310XT. It does not display the same messages as the 910XT (though I wish it did).

I have the same setup, aluminum G3 clincher, 310XT, and I don't usually do any calibration at the start. Sometimes if I'm on the trainer, or it seems my power readings are too high or low, I'll stop and go into Settings > Bike Settings > Bike1 > ANT+ Power > Calibrate, and it'll give a number somewhere around 512 +/- 2.