Just go with national / global pool for the denominator. There are about 4 million high school graduation age children in the usa. Probably about 80-100 million world wide. About 20000 ivy league spots per year. So you need to be about one of 5 in a 1000 in the usa. Maybe 1 in a 1000 globally after adding global equivalents of us Ivy League schools. basically top 0.1 to 0.5 %.