Just a further note on Nunatak fill weights.. Now, don't get me wrong, I love Nunatak gear (I have plenty of it), and there's no doubt it's warm and ultralight - even if it is very expensive.
But, for example, look at the Skaha Vest published weights in Pertex Quantum, size M:
Fill weight: 3.75oz
Total weight: 5.5oz
From this we can deduce that the fabric weight is 1.75oz... which I don't really believe, given that the lightest wind vest (admittedly full zip) I can find is 2.2oz, which is only a single layer of fabric. And at any rate, the vest in reality is most likely heavier - my stock Skaha Plus Vest in Quantum 0.8 is 7.5oz instead of the listed 6oz weight. Is the extra weight more fill, or is the fill accurate and this weight the real fabric weight? Or is the fill in fact less than listed, who knows?
So the problem with calculating warmth based on down weight is that you have to trust the published fill weight (if there even is one).
My clusmy way of measuring warmth (assuming fabric weight+type is similar): how hard is it to stuff into a sack :) It's not totally ridiculous - I used to have a custom Minimus Ultra Vest with no pockets and a stud on hood, but replaced it with a Skaha Plus Vest (mainly for the better hood). The Skaha takes more stuffing into the same volume, therefore it's warmer (and personal experience seems to bear this out, but frankly I don't trust that).