You can get a "Rheostat" that will make your fan a variable speed fan from the hardware store. Its just a knob that will control the speed of the fan. Seems like consensus is drop it don slowly, i bet if you drop too much too fast you'll end up shocking them like pulling the lid off the clone tray without opening the vents first. Drop ten percent every few days till your in the 50-60 then drop it off even more in the last mont of flower running around 40 percent. You can also get a hum meter that can kick your fan on, some one smarter than me like @Moe.Red knows the name. They make em' for green houses