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If you want to add a hair light inexpensively a better approach would be:
1) Get a sheet of silver backed foam insulation or cover a sheet of cardboard or foam core with aluminum foil. Suspend it from the ceiling just behind the subjects out of the frame. It will reflect the key and fill lights back towards the heads creating a hair light effect.
2) Any cheap hot shoe flash with an optical trigger can be used. Create a "snoot" out of aluminum foil and either aim it directly at the subject, or with a low white ceiling put it on the floor aimed up creating a spot of light on the ceiling between subject and background. Bouncing a spot of light off the ceiling is very effective strategy for adding a "hairless" light - adding an accent to bald guy's noggin.
You gain experience by observing results and if you do some systematic experimentation one variable at a time you will grasp the cause and effect faster and with more clarity.
For example if you want to see how fill position affects results set up a subject with fill over the lens to start and key light at 45° from the nose of the subject. Shoot a baseline shot that way, then using a string on the fill light to keep the distance the same move it around to the side 15° at a time, taking a shot with fill 15°, 30°, 45°, 60°, 90° off axis until the fill is pointing at the ear on the side of the head. With that 15 min. systematic one variable test you'll learn more about fill position that months of random shooting will teach you because its easy to see the cause and effect of just the fill when its the only variable. It's not a matter of one approach being "good" and the other "bad" but rather if the look the fill position creates matches your goals for lighting the shadows.
You should do that with all your lights. Keep fill constant, move the key light around the subject, observe where the highlights and shadows fall and whether or not it looks natural and flattering. Set up key and fill, leave them alone and just move the hair light around and observe when it adds separation between background and foreground and when it becomes a distraction that pulls attention away from the key light front of the face.