How warm should a down jacket be in a layering system?
I'm building a layering system for cold Colorado winters where I will be going from strenuous hiking to standing still in below freezing weather to take photographs. In full my layering system would consist of: mid-weight base-layer, adjustable mid layer, down jacket, waterproof shell.
Any recommendations on how warm of a down jacket I would need for below freezing to -10 F (-23.3 C) weather with that layering system?
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The answer really depends on how long you plan to be standing still. If you plan to be standing still for extended periods of time, say 10 or more minutes, I would suggest a warmer coat. If you're only going to be standing still for shorter periods I would suggest a lighter down jacket.
It is important to note that a higher fill does not necessarily mean a warmer jacket. As for exactly how warm you want it to be depends on what you prefer, some people wear really warm coats in weather that others go with just a sweatshirt in, so an exact number is very hard to give due to the differences between people.
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The warmth of clothing is often defined in units of CLO. A 1 CLO ensemble of clothing is suitable for sitting in an office (or more specifically having a metabolic rate of 60 W/m2) at 22C or slow walking (or more specifically having a metabolic rate of 60 W/m2) at 12C for an average adult male. To be comfortable at -23C (i.e., -10F) at rest you need 6 CLO.
It turns out that high quality down provides about 1.7 CLOs per oz/yd2. Since your body has more than a sq yard of surface area, the number of CLOs you will get from an ounce of down is much less. In fact, the average male body has a surface area of 2.44 yd2 and this means an ounce of high quality down gives you about 0.75 CLOs regardless of if you spread it out all over your body like a sleeping bag or on half your body like a jacket. To get 6 CLOs, you need approximately 8 ounces of down. This is ignoring your base ensemble and assumes you are at rest when taking the pictures. It also assumes you are a perfectly average male.
As you need the layering system to handle -10F to +32F, you may be better off with either two jackets, a jacket and vest, or even better, in my opinion a jacket and pants. As long as the total fill weight is around 8 ounces the average male will be fine.
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First of all, you shouldn't consider wearing a down jacket under a mostly non-breathable shell. Down loses most of its insulating properties when damp or wet and, assuming strenuous hiking, having it inside your perspiration saturated outer shell will only degrade its performance over the course of your sortie, to the point of becoming almost useless.
If anything, you need to put the down jacket over your shell. As an alternative, if you really want to put it under and don't own the jacket yet, get a synthetic fill jacket instead. Synthetic fibers retain their insulation much better when wet as they don't collapse like the down plumules do.
Secondly, your description of strenuous hiking is incompatible with a layering system where you're wearing a down jacket permanently. The key to being able to stop for exented periods of time is to stay as dry as possible. Over-insulation will promote excessive perspiration and even though this isn't a problem while moving, the moment you stop and cool off, moisture-logged insulation will make you miserable.
Unless you're doing some really leasurely hiking, operate at extremely high elevation where one step takes 5 seconds to recover, or are doing technical mountaineering with less aerobic performance, a down parka should be carried in your pack until you stop moving.
The main thing is, if you're going at close to 100% effort, you don't need much insulation and you should keep it for when you go 0% effort.
As an example, and I don't pretend to represent the norm, my usual setup doesn't change whether it's 32°F (0°C) or -4°F (-20°C) while moving. I wear a heavy base layer and a heavy WB400 softshell (closer to a true hardshell, it's a winter fabric). My hardshell and down parka are in my pack and only come out if I'm facing extreme wind or taking an extended break. Some days, I won't even use them and they effectively are just part of my emergency kit.
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