How to space recessed lighting?

I need to know how to space the lights I bought this afternoon. We are finally building our upstairs after living in the basement for almost forty years. It seems we need more light as we age and I thought adding the 6" canisters with the reflective trim (in addition to the fan with the four lights) would be a good idea. I think I read that a canister placed in an 8 ft ceiling would light a 8ft diameter space but now I'm not sure. Could anyone help me with this? I need to give the builder (DH) the plan by Monday. (My web searches haven't helped.) Doris