I have this guy: Amazon
I have an element installed in it. The locknut on the inside butts up against the indentation so it can't spin (makes screwing the element in easier since I don't need to hold both the nut and the element).
I drilled the hole smack in the vertical middle of the lower flat area. I could have moved it down a touch possibly. I figured the middle was the best bet because the bottom curvature and the above indentation.
What I'm not sure of is that if the indentation is lower in smaller pots (or different brands).