Down fill weight percentage of total weight

Sure, it's a factor, but what do you mean by "comparing"?

Warmth?
Weight?
Fit?
DWR?
Construction?
Packed Volume?
Price?

What conditions are you addressing and what are the candidates?

I suppose construction efficiency(?) Maximum down in the minimum shell.
Of course in the real world you would choose whatever suited your needs best.

How much could you rely on just fill weight and overall weight to deduce which jacket had a better warmth:weight ratio? Would it be completely irrelevant without knowing fill power, construction, shell material, fit etc?

Just thinking aloud here. Might be going nowhere.

i would think you could only compare sewn-through to sewn-through and box-baffles to box-baffles.

I assume the box-baffle construction weights more as it would have more fabric, but it would also have more warmth for that weight.

you can add extra weight by putting in pockets. zippers. trim pieces.

I like a zipper down the front so you can adjust warmth, even though it adds a little weight

if it's thicker, you can have a higher percentage of down. If it's an ultra-light sewn through, the face and lining fabric are a big percent of the total weight

K: From one of our resident clo experts (R Nisley), sewn through v baffling doesn't seem to matter much until fill weight gets above 6 oz
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=26244

Well, in another thread RN did his own test which showed that up to 9 oz. there was no significant difference between sewn-through and other forms of baffling. More interesting to me then that number, is the evolution of those differences for different fill weights

Maybe I am too simplistic, but the ratio of fill weight to overall weight is my first consideration in evaluating down pieces, whether it is choosing a sleeping bag or a jacket.

UL manufactures, to save weight, all compromise some place, or in some combination of places. Fill weight, shell material, sizing, cut, features. I decide what I need a particular piece to do for me, then I make the first cut based on the fill weight to overall weight ratio. The first thing I need from a down piece is to keep me warm in a given range of temperatures and conditions. I do not want a down piece that cheats on the amount or quality of the fill.

Once I have a short list of pieces that are not cheating on the fill weight, I look at what shell materials are used. Lots of companies will compromise by using cheap shell materials. Since I would wear the down piece under a shell in snow or rain, I am more comfortable going with a lighter DWR treated shell like Pertex Quantum then something beefy like Epic. Then, what is the sizing of the jacket? Lots of manufactures use very athletic sizing to make weight. The Large is really a Medium etc. Then, what is the cut of the piece? Does it ride up really high, like a belay jacket? Is it unnecessarily long? Finally, what features do I need or want? Does it have to be full zip, have pockets, have a hood? Finally, how much does it cost?

I just went through this process over the last 3 months and have, after some buying and selling, settled on a Nunatak Skaha Jacket and a Nunatak Skaha vest. The jacket is a full zip with pockets and is for winter hiking. The vest has a half zip and no pockets, and is for my Shoulder Season kit, to give me an extra measure of comfort and safety in the PNW into October. Nunatak is really expensive, but I got both pieces on the Gear Swap Forum for under $300.