Use the rule of thumb, never take off more than 1/3 of the height at one cutting. I "top" all of my lawns and rarely measure how tall it really is because I know there are benefits to leaving it tall. Cooler soil temps., shades the soil preventing weed growth, longer root system, stronger overall stand of turf. Books will give you recommended cutting heights and thats fine, but they tell me to mow bermuda at 1.0 to 1.5 inches!!! I don't think so, I'm not mowing twice a day at the golf course or baseball fields.
"CUT IT HIGH AND LET IT LIE"
HOMER