I was under the impression that Garmin used the barometer to augment the GPS elevation, or is it the other way round, I can't remember. Anyway, general guidance is to acclimate the Garmin for a few minutes before you actually start your ride. I have heard every number from 3 to 10 minutes. If you are leaving from home, put your bike on the porch before you put your helmet and shoes on, fill your water bottles and fill your pockets with whatever it is you carry in your jersey pockets. That should be enough time to acclimatize the Garmin.