To truly layout light fixtures well, you need to take into account many things like placement of windows, location of cabinetry, the layout of the furniture and artwork, the height of the ceiling, and the type of light fixtures and lamping. A 4 to 6 foot spacing between fixtures is fairly typical for an even spread of light in a room with an 8 to 10 foot ceiling, but this can vary with the photometric characteristics of the unit you install (see below for typical photometrics diagrams). It's also important to know how the room will be used and furnished, as an even spread of light is seldom the most beautiful. I'd suggest that you start with a furniture plan, figure on the lighting needs and desired moods, then select your fixtures and placement.