Depends on the turrets. With gauss, I'd lean towards using a 1:1 (BFC:Turret) ratio where the turret contains four 5HS GCs. For other beam weapons, I'd personally favor a 1:2 ratio using quad turrets. Most of the other posters have more practical experience than me, but those are the impressions I've developed while reading the boards. Note, however, that one's designs should depend on what you're fighting against; the previous advice is primarily aimed at PD being used in an anti-missile role and wouldn't hold up as well against, say, a swarm of meson equipped fighters.
No experience with missiles, so can't comment on that angle (My first game is using the no missile challenge, heh).