A lecture I attended given by Alan Fletcher,. He said that if he wrote a technique Book he would not use the term ball flat but would sustitute it for inside edge of toe. Thats what I have done ever since. When you think about it, if you are bringing your moving foot from behind, getting to a high instep as it draws level I find myself on the inside edge, so why alter it. I am talking more towards Rumba here. The Cha is basically the same though.