Regarding ratings and warmth, I treat the manufacturers rating as a more of a vague suggestion. It gives you an idea of how warm the bag is, but if you really want to know how warm it is you need to focus on the amount & quality of the down.
After using quilts for the past couple years, my personal rule of thumb for average girth QUILTS is:
40F – 7oz 800FP down
30F – 9oz 800FP down
20F – 12.5oz 800FP down
This assumes you're an average sleeper, on an R-2 to R-3 pad and wearing light-medium baselayers.
With a sleeping bag, you've got more area to fill with down (wider + hood), so I find you generally need about 30% more down to achieve a similar warmth. My personal rule of thumb for bags is:
40F – 9oz 800FP down
30F – 12oz 800FP down
20F – 16oz 800FP down
When I look at the specs on Zpacks sleeping bags and see 10.5oz of 900FP down (equal to 11.8oz of 800FP) in their 56" girth regular length hood-less bag, that strikes me as being a little light for 20F. If all that down was in a quilt with a girth around 52", then it would be close to my 12.5oz goal for a 20F quilt. I personally would rate 10.5oz of 900FP down at about 23F in a quilt and at about 26-30F in a sleeping bag, except the Zpacks bags have no hood to share the down with, so I would say 25F would be my personal expectation.
Consider that Western Mountaineering puts 16oz of 850FP down in their 20F rated Ultralite bag, and it has a fairly similar girth. Also, WM's MegaLite bag uses 12oz of 850 FP down for it's 30F rating and that's more down than Zpacks 20F bag. Of course these have hoods and the Zpacks bags don't though.
Thinking beyond just the temperature rating of the bag, two big things to consider are:
1) What other gear will you be carrying that you can wear inside if needed? I've always got my Ex-light down vest with me in the summer and in the shoulder seasons I've got my MB Alpine Light jkt and down pants. I recently switched from a 25F quilt to a 35F quilt because with the 25F quilt I wasn't able to take advantage of these layers I was carrying anyways. I can take my 35F quilt to 30F in the summer with my vest, and I think I'll be able to take it to 20F in the shoulder seasons with the big parka, down pants and warm socks. Think about what temperature rating you want your bag/quilt to be capable of handling, and then ask yourself if you were on a trip where encountering this low is a realistic possibility, what other warm layers will you already be carrying? Maybe you want to be able to handle 20F nights, but you only need a 25F bag because you carry Cap 3 tights, down socks and a Patagonia down sweater?
2) Adding more down to your sleeping bag is by far the most efficient way to add warmth, aside from using other layers you've already got with you. The shell weight of a sleeping bag (everything but the down) doesn't change much whether you've got a 40F bag or a 20F bag, so if you opt for 2oz more down, it might only cost you 2.2oz total. Conversely, down clothing is rarely more than 50% down as a percentage of the total weight, so making up for a cold bag by adding a bunch of warm layers gets heavy. Absolutely you want to take advantages of the layers you're carrying already, but you also don't want to wind up in a situation where you have to bring extra layers just to wear them at night because you didn't get a warm enough bag. If your bag isn't warm enough and you need an extra 2oz of down, it would have only added 2 to 2.5oz to the bag, while adding a down garment with 2oz of down will likely cost you 7oz.
So the bottom line is to absolutely use the warm layers you're bringing anyways, but don't get too cold of bag so you need to bring even more layers than you need outside the bed.
Freezing sucks….don't cut it too tight either.
