1. first you need to know your flash's full power, so when it is set at 1/1,
what aperture you can use at iso100, you probably have to memorize it. For
me, a canon 580 EX II, full power I can use f/5.6-f/8 depending on how close
I put the light. So to answer your question where to start, I will start at
1/4, so I have 2 stops to go up and rooms to go down as well.
2.The reason to place a light, is to create/control shadow, its darkness,
and its shape. When you have a light coming on the side, if you want a 1/2
and 1/2 contrasty look, you don't need to fill
3. when shoot back light, I will always use light in the front, at a very
low power. I find reflector is hard to control when shooting solo, so I use
flash.