The math around fill power and loft and jacket comparison.

Possibly the clo/inch is different for different fill power but I have no idea which is bigger or by how much. That could effect your calculations.

The inner and outer lining provide a little warmth. With jackets that have so little down, this is more of a factor.

Lower fill power is supposed to resist humidity more, so the 800 FP might be better in real world conditions.

I have heard complaints that the Ex Light down jacket has so little down in it that it doesn't produce very much warmth. I forget where.

BGT has some reviews that might be useful http://www.backpackgeartest.org/reviews/Clothing/Jackets/

yes… you would have to factor in the amount of cloth too. However, in terms of heat, I don't think it's giving you much.

Maybe take in the total weight of the garment too.

The Ex Light down jacket definitely wasn't as warm for me.

Also, I was reading that the higher the fill power the more fragile it is… so while it's lofty and warm when you first buy it that might change a bit rapidly.

perhaps 800fp is superior in that it might last longer (so reducing the cost of the jacket).

Your approach is generally a fair one to use. I think it's far better than crude measures of loft. There are minor issues, such as hoods as you mention and differences in cut, sizing, distribution etc. Higher fill power down is more susceptible to losing loft from humidity, but I don't think it's necessarily shorter lived.

And yes the Mirage is awesome. If you find it in medium please order 2 and send one to me.

I created a spreadsheet with all this data:

https://docs.google.com/spreadsheet/ccc?key=0AkBAy3nnEh65dEtpNlpzSXJpa0NLMDNlZFZpa1I1Wmc&usp=sharing

I only added the down jackets that looked like ones I would buy.

Montbell is the only vendor that you can do this with. None of the other vendors publish their fill weight. So they're not getting my money.

Though with the Montbell data it seems you can multiple the weight * 0.35 (approx) to get the fill weight.

The EX LIGHT DOWN JACKET MEN seems to be more than 2x some of the other jackets in terms of fill volume.

This data REALLY does make the Mirage look amazing.

The only thing that makes it more confusing is whether 800 fill power is a better investment as 900 fill power is more fragile and degrades quickly. I would need some sort of data to determine how quickly it degrades to 800 fill power.

BPL collected data on the down fill of quite a few jackets in the State of the Market Report a couple years ago. You can view that here and add these to your spreadsheet:

http://www.backpackinglight.com/cgi-bin/backpackinglight/ultralight_3-season_down_jackets_sotm_2010_part_2.html#.UwIPe3lIjfY

Montbell also makes the "Guide's Parka" which looks like the same thing as the Frost Smoke but without the more durable fabric patches, so it should offer a higher down %. It's not officially offered in N. America but you can find them on eBay:

http://www.ebay.com/itm/New-Mont-bell-Men-Ultra-Light-Down-Conseal-Zip-Guide-Parka-1101384-S-M-L-XL-s-/231156565003?pt=US_CSA_MC_Outerwear&hash=item35d201440b

Backcountry Gear has the Montbell Mirage in Medium (says Dark Navy ships in 24 hours).

I found it at a local store in stock in medium.

http://www.downworks.com/downworks/Welcome.html

They have 3 other jackets in stock in medium in the three other colors but I got the last gunmetal grey :-P

… so I guess if you want one call these guys.

and it was $309 so the normal pricing. But I guess it's in CA so I had to pay taxes on it (boo!)

"Montbell is the only vendor that you can do this with"

Maybe I am reading this the wrong way but for example Western Mountaineering does post the fill weight of their jackets.
The fill on Quick Flash is around 2.5 oz, the hooded version has 3oz.
Fill weight is also listed for their other (heavier) jackets.
Feathered Friends list the fill weight of theirs as Sierra Designs also does.
For example the 14oz SD Stratus Jacket has 3.5 oz of fill.
(those are the only ones that I looked up…)

> Western Mountaineering does post the fill weight of their jackets

see… this is why I avoid using superlatives.

the vendors I picked all were not using fill weights. Kudos to Western Mountaineering!