Anything between 4 and 15 percent can work. At the bottom of that range you get more of a speed-training effect, while at the top of the range you get more of a strength benefit. But I’d put the sweet spot at 7-8 percent. I recommend adding 10 seconds to each interval to account for the time it takes the belt to accelerate. Also, whereas in outdoor hill workouts one usually jogs back down the hill for recovery, on the treadmill it’s more practical to keep the belt at the same incline and just slow down (to a walk, if necessary). Enjoy!