Personally, shooting film, I'd be pretty thrilled with 3:1 or 4:1 shooting ratio. There will likely be shots that don't take more than 1 or 2 takes, giving you more takes on critical shots. You've just got to be a stingy bastard when it comes to doing takes, trust your cam op, and plan, plan, plan.
I listened to a Dov Simmons(sp?) seminar once, and he suggested only budgeting for 2:1, since a low budget filmmaker doesn't have the time to shoot more than that, let alone the stock. Shooting 15:1 isn't just going to jack your film cost, it will affect everything, because you'll be shooting much longer, paying your crew longer, have a longer shooting sched, etc.
Heck, I don't think I've ever shot 15 takes of anything! Not with film or video. Then again, I am a hack.