Are you sure you want to bother with a stuff sack for the sleeping bag? Like someone said earlier, all it really does is turn it into a brick that forces you to leave unused space around it. I know - the stuff sack protects it from getting wet. I always thought that, too - until the day I asked myself, "When was the last time the inside of your pack got wet?" Then, like a good little control freak, I answered, "Never." I quit carrying my bag in a stuff sack at that point, and found that I could stuff the bag into all the empty corners at the bottom of my pack, use the bag to hold other things where I want them by forming the bag around them, and even maintain the volume of my pack for smaller loads (weekend, warm weather, etc.) by not compressing it so tightly.
Especially if you use a poncho, and probably if you use a pack cover - has anything in your pack ever gotten wet because the pack let in water?
And, if it has, consider using a large garbage bag as a pack liner to keep everything - bag, clothes, food or whatever - dry and separate from a wet tent you may need to carry inside.