The best height for me to hang a strop is halfway between my navel and my nipples. This gives me a comfortable stroke that feels fairly natural to the elbows and wrists.

Something I've done with a paddle strop is put the handle end on a counter and support the wide end in my off hand. That allows it to rock the way a hanging strop is free to. I agree with the original post that a stropping surface laying flat on a leg, a tabletop, or a counter feels unnatural. I agree that it's hard to get a good motion going relative to that position.