We use two BV500s, or only one for shorter trips. Pain in the neck to fit in a pack!

As for weight, I usually add up the net weight of each food item. The exact weight is off by the weight of the packaging, but it's a decent approximation. Unless you're carrying lots of really dense food like peanut butter, the bear can might turn out to be one of the least dense items you're carrying, even if it's one of the heaviest in terms of total weight. For example, those prepackaged freeze dried meals are packed in air, so they tend not to be very dense. 

If you're sure you'll have plenty of water available, I'd consider carrying only one liter. That's 2.2 pounds. Carrying two containers gives you the flexibility to fill and treat one while still drinking from the other.

I suggest loading your pack the way you think you want it, and taking it on a day hike, preferably one on terrain similar to what you'll be backpacking on. If there's something wrong with the way it's loaded, you'll know within a few miles!

In the end, what matters in how your pack is loaded is how well it works for you. I've seen all kinds of weird arrangements, including the sleeping pad between the pack and the person's back! No idea why, but it seemed to work for him.