Normal planting rates for new grass with fescue are 5 to 10 pounds per thousand to begin with, so 15 pounds per 1,500 square feet equals 10 pounds per thousand. It's at the top of the normal seeding rate, but it's still okay. If some smaller areas go up a little higher, it won't be a problem. Too much turf seed causes undue competition for resources such as electricity, water and nutrients, and turf seedlings struggle as a result.
Too little seed leaves grass thin or bare. Always follow “best practice guidelines” for planting turf seeds, including site preparation and good contact between seed and soil, and follow recommended seeding rates for lush results. If it's too dense, the seedlings don't root well and a lack of air flow could make them more prone to disease. Grass growth occurs in seasonal cycles, which vary according to the types of grass common to different regions.
However, remember that re-seeding only requires about half as many seeds as laying a new lawn and there are a couple of grass varieties that actually require less seeds than that, so don't overdo it if you already have some grass to work with. One of the most common problems lawn professionals see is when homeowners misjudge their real turf areas and overapply turf seeds or other products, such as fertilizers and herbicides. If you try to plant cold-season grass in early summer, for example, it probably won't work out very well, no matter how many pounds of grass seed you put in per square foot or how much fertilizer and hay you put on top to promote growth. In general, the measurements given for turf seeds per square foot in any grass seed breed only apply if you sow them during the intended season and with the correct humidity, soil, light and temperature conditions for the seed to germinate properly.
Obviously, the variety of grass will greatly affect how many seeds you'll need per square foot, but what I'm talking about here has nothing to do with cultivating grass, but rather with the type of seed itself. Depending on where you live, what your general climate is, and what specific variety of grass seeds you use, you should generally plan to plant at least 2 to 3 pounds of grass seed per 1,000 square feet; generally no more than five pounds, although there are some exceptions.