First:
- Attach the camera to ALL the mounts you want to use (including the one that sticks to the helmet).
- Put your helmet on.
- Hold the mount/camera in the EXACT position it would be in if you glued the mount to a given location.
Then line the camera up carefully, and be sure that you can get the camera in the position/angle that you want if the mount was to be glued to that position on the helmet.
If all is well, slap that thing on there, unless it's a thermoplastic helmet of course, apparently adhesive mounts can damage them???? (So they say, I really have no idea
)