are you asking about the hot part of the element and the oposing wall? I think 1/2 inch would be enough. If it is closer than that you can manipulate it a tough when hot and move it further.
You can bend them a little. Put some water in it, turn it on, and use some pliers to lift it up a little. You should be able to make clearand 3/4 or so by doing that. I have two elements in mine, and they sit about 3/4 inch apart at the tip. Works fine, but has hummed a time or two when running.