very good advice from low rdr as well.
What I do is I have some cardboard sheets cut 2' x 4' painted flat white and I use them to contain the light onto the plants until they fill the grow area. Lighting a 4' x 4' space when the li'l plants can be cuddled in together in a 2 x 2 space with the reflector boards will make the most of your lighting dollar. I also use some reflector sheets over the top as well. It's cheap, it works.