I am not going ot try to tell you what is safe, but I will tell you my pool is a 20x40 hopper style deep end at 9.5 ft. (10'x10' flat floor area) The somewhat shallower angled slope up to the shallow end begins at 15 feet from the edge of the pool. There is a pool rule about not diving face first, which people have broken numerous times over the years, and as a result more than one person as kissed the bottom usually around the 5-7 foot deep part of the slope. Including one incident with a teenage girl whose braces cut a band aid size hole through the liner, and resulted in an emergency visit to the dentist, thankfully she did not loose any teeth. Also when diving feet first it is easy to touch bottom and push off to return to the surface. How much longer does the deep end need to be, I don't know, I do know even a couple of more feet would be helpful in lowering rates of kissing the bottom collisions, 5 feet would be better, but it might take 10 or more feet to elimiate it completely, which simply does not work with typical residential sized pools if your going to have any shallow play end. If I had it to do again I would probably do a staged shallow end with 2-3 ft wide 3 ft depth, then a slow transition to 4 ft. and the remainder of the shallow end around 4 ft.
Ike
p.s. I can't help wonder if this diving board vs shallow end vs slope angle delima might best be solved with an odd shaped pool and perhaps a corner area mounted diving board.
Ike