do you know how high? Enough so that the thigh is parallel?
From the article:
Finally, he concluded that the ideal position generally occurred when the athlete was standing on the toes of one foot with the other foot flat on the bench and the top of the raised thigh parallel to the floor.
That seems to suggest that the working leg is slightly above parallel when the lifter is standing flat footed, since it is parallel when he is standing on his tip toes.
That's an academic point for me - when I do step-ups, my working leg is below parallel. I try to take long steps so my glutes get the brunt of the work. Sort of a bastardization of the step-up and lunge. Taking long high steps would be an invitation for my kissing the floor.
As he began to experiment with different heights, he soon realized that he could achieve complete development of the thighs and hips by using varying bench heights, depending on the needs of the individual athlete. Being well-schooled in anatomy and physiology, he understood that the higher the bench, the more stress would be placed on the hamstring muscles on the rear of the thigh. Conversely, he understood that a lower bench would result in more work being required of the quadriceps muscles on the front of the thigh.
Nothing is said about varying the length of the step to work different parts of the legs and hips, but based on DOMS, I think that I'm doing the right thing.