It depends on the pitcher and her level of experience. I always set it knee-high on the pitcher herself. For someone new to the drop ball I'll set it four or five feet in front of the plate. For a more experienced pitcher I will move it closer to the plate -- sometimes only a foot or two in front. It's tough to get over the string and get the ball down on the ground at the back of the plate, but if she can do it that's a tough pitch to hit!
Ken, how do you set your drop ball practice "string"? was wondering how you did it?