1) Its difficult to properly align your head. There needs to be some kind of marker indicating the center of the head bone to help with aligning the head.

2) Seeing past a skirt or dress to get your legs right is pretty much impossible without a mirror. So, proposal: When doing calibration, spawn a local-only mirror in front of the user's avatar at a meter or two distance. Maybe even cull/shrink other users for the moment so people don't get in the way.

3) There should be a manual "recalibrate" button so I don't have to swap back and forth between avatars to fix a bad calibration.