There's no formula. However, a program like OnTarget2 shows a trajectory curve and from that I can get where the 2 points are. I got the idea from an article by John Dudley on the method. He stopped paper tuning his bows a while ago and found that when shot thru paper had better holes than just trying to paper tune.

I can't attach the the file thru email. Here is the link I got the article from.

http://www.dudleyarchery.info/articles/usarcher1FT.pdf