I would not use cardboard for a reflector as I would be concerned of a fire. An aluminum pie pan or any thin shiny flat piece of metal would work good. You would want it within 1 inch of the lights, just enough to allow light to escape. Curved around the lights if you can. A large clamp reflector works great, is what I use.
Those cfls are good to maybe 7 inches, the 45 watt ones. Putting them in a splitter can make them double the light penetration if the beams of light intersect, so 14 inches maybe.
In flower you want that light within 2 inches of plant and 1 would be better if they can take it.