The first and most important rule for mounting a light on your helmet is that it must break away readily when you crash or catch an overhanging obstacle. If it does not, you risk having your neck jerked when it snags on the pavement or tree. Besides jerking your neck, that can add to the g's of the shock to your brain when you hit pavement.
There is no standard for how easily the light should detach. The CPSC standard says it should "readily" detach during normal lab impacts. But "readily" is not defined. Few helmet or light manufacturers have given enough thought to their mounts. Only one helmet manufacturer we have spoken to provided their lab test levels, Uvex, proving that they actually have an internal standard. And the light manufacturer Jet Lites has a standard requiring their mount to break away when loaded with a 5 pound weight.