I thought of using my Samsung Galaxy S2 Smartphone. I was surprised to learn that the frame rate on that particular phone may vary under some circumstances. Maybe it only slows down (<30fps) in low lighting levels. ? I would check that point for any phone video camera before assuming that it is an accurate 30 or 60 fps.
Scale for calibrating distance-
Another consideration - most smartphones have very wide angle lenses. The magnification changes across the image.
There are some golf smartphone applications that you might want to check.
Good luck, it looks like a great, fun project!