Richard,
I would strongly suggest 'grasping' fill weight.
The first reason is that loft and warmth are only loosely correlated. This may be in part due to the fact that loft is very hard to define, since a down garment tends to have some sort ridge like surface due to the baffling.
Granted, fill weight alone is not always correlated very well with warmth either, but it seems to be closer.
Most importantly, fill weight is easy to find(most decent lightweight manufacturers list it), where as loft is not, and as mentioned above, is not that easy to measure.
I would suggest about 6 oz of down for a winter parka. More is always nicer, but off course weight and price goes up.
I'd echo the comments about a hood and add that you want to make sure it's long enough. Since you will be using this while sitting, the last thing you want is an exposed back.
Many lightweight jackets are fairly short, so make sure you hunker down with the jacket as if you were cooking and check coverage.
By the same token sleeves should be long enough.
A more wind proof shell will add warmth, but add weight and reduce breathability. Baffles will add weight but also warmth. For me, a simple elastic baton is sufficient, since I don't ever feel the need to vent this jacket, other than unzipping it.
I think it should be just big enough to fit over your 'action suit' ie the shell and base layer you'd be wearing while moving. Not much bigger as overly loose parkas tend to loose a lot of warm air to the belllows effect, and make it hard to put a shell on over them if needed.