As far as composition, I would place the camera in a dark area (room, under a blanket, etc). Aim it at a brightly lit subject area and cover your back with a sheet of tracing paper or some such. Let your eyes adjust and you should just be able to make out some features in your subject area and through comparison between the projected image and your subject area what your coverage will be, albeit only roughly. PH is very much point and guess until you become familiar with your camera.