Permissions: how should the permission levels for the users of Calendar Browser be set?
Permission levels should be set according to the privilege your Calendar Browser users require.
Typically most Calendar Browser users would require Editor or Publishing Editor to add/remove/change bookings on a public folder calendar. But this could be fine-tuned according to the level of accessibility you want with your users.
The owner of the Calendar Browser public folder usually grant/sets permission level for other users in case of public folder calendars.
With respect to direct booking mailbox, the mailbox owner has to set/grant permission over its calendar.
This is a brief description on the privileges/permission levels available: