Hi Scrichlow! I am 5'2" also and I use 5 risers under my high step (14 inch height total). I don't have any problems with that height but that does not mean that is the correct height for you. You may decide that lower is better for you.
Some things to consider when deciding on height are that you are balanced and able to maintain proper form while doing leg presses (the exercise that uses the high step at its highest height). You should also not experience any pain in your knees. Another thing is if you find yourself struggling to keep up with the rep speed or having difficulty lowering down or pushing off the high step without awkwardness, then lower your step height.
Hope this helps!