Your mowing Tall fescue and you want to mow it at 6 or 7 inches? Is this for a lawn or are you just cutting it as a field? I ask because you should mow tall fescue at 2 inches during the fall and spring, raise to 3 to 3 1/2 inches in heavy shade and during the heat of the summer months.
I agree with Rick on this one. Fescue does slow its growth in the hot part of summer seeing as how it's a cool season grass, but cutting it 6 to 7 inches seems to be more for a livestock pasture than for a lawn. One problem I see with cutting your lawn that high would be that it would attract rodents and snakes. Not to mention an increase in insects (Mosquitos, ticks, chiggers, etc). If you feel that you need to cut it at that height, then you may be better suited purchasing a tractor with a bush hog. If you want a green lawn throughout the summer months, then you should look into removing the fescue and replacing it with a warm season grass. This would be somewhat costly and very time consuming, but you would probably have better luck keeping your lawn green during summer.
I live in a similar type climate as you (west Tennessee). We are somewhat limited in the types of grass we can grow in our region because our summers are typically hot and dry as well. Grasses such as Kentucky bluegrass don't grow well in our area due to the low amounts of rain fall during summer months. Around here the most common lawn is Bermuda. However, I did have zoysia in the lawn at my old home, and it handled the climate well. It made for a very beautiful lawn.