At least 60m away, and 5m high.
I doubt that there is such a thing a bag that bears absolutely can't smell through. Bears and other animals with rhinariums (wet nose) not only smell better than humans, they smell in a different way, they can literally taste odours. A bears sense of smell is roughly 2100 times more sensitive than a humans. Even if you got a fancy odour-proof bag to stick your food in, the bear would still be able to smell the bag, which could be enough for it to want to come take a closer sniff. Up close, I doubt there's anything a bears nose couldn't penetrate. Your greasy hands that handled your food also handled the bag, a bear will be able to smell that, and will probably assume there will be more of that tasty smell inside the bag.
There's not much you can do to prevent a bear from smelling your food, so you have to take measures to ensure that the bear isn't attracted towards your sleeping area. I live in bear country, and personally, I don't use any kind of fancy bag to hang my food (a plastic bag is good enough), I just make sure it's a good distance away from my tent, and hung at least 15' off the ground between two trees where there's no way a bear could reach it. In my opinion, it'd be better not to use a smell proof bag, I'd rather the bear was following his nose away from my tent site and kept distracted all night far away from where I was sleeping rather than not notice the cache and come sniffing around our tents in hopes there might be scraps.
It's important to keep your cooking area, and toileting area a good 60m away from your tent as well. Small spills, boil overs, even just touching the log you're sitting on with the same hand you barely wiped your mouth with will leave a scent.
A dry bag will work fine as a food cache, these's no need to go buy one of those gimicky expensive bear bags. As long as you build a good cache a couple minutes walk away from your tent, and put absolutely everything you have that carries a scent with it (including lip chap, tooth paste, deodorant and chewing gum) you'll be just fine.