Actually, I always learned that 25 was the starting point and work your way from there, but a couple of dudes at a LBS were adamant on 5 and they seem to get a lot of people going in for fittings - always when I am in there there is someone - and a pretty authoritative angle measuring stick so I figured they were right.

Just for reference I am somewhere between 25-30 using rudimentary measuring methods, hence the give-or-take numbers.