When it comes to how large of a space a fly light will cover, the fly manufacturers will tell you there is no way to determine that.  Here is a installation summary directly from Gilbert, the company that makes the Gilbert 2000GT Flying Venus:  "File any published "area covered" claims in your wastebasket. At best, they are, generalized, rules of thumb. At worst, they are misleading advertising. They certainly shouldn't be used to compare traps. It is nonsense to say any flytrap will control insects within "X" square feet of space (even indoors) without, at least, specifying the species of insects concerned, the type and condition of the facility, the surrounding environment, and management's level of concern. The distance at which an insect responds is determined by lamp type and trap design, of course, but also by the number, visual acuity, and nature of the specific insect." We recommend purchasing one fly light and after installation and how it controls your situation determine if you may need additional lights.