I would recommend not measuring chest and jacket to check fit. It is very hard to know how the jacket will drape when actually worn and having the down loft up.
I find that with thicker down garments I can get away with a baggier fit, because the down lofts up and 'balloons' inward, taking up the empty space. This allows for layering too.
So here is my guide:
Assuming you will only bring this jacket if it's below freezing.
Typically you would put a jacket like this on at rest stops and in camp, over the top of your active clothes(skiing/climbing etc. outfit)
Layer it over a base-layer. Is there so much space inside that air is swooshing in and out as you move? This will make the jacket less warm.
Next, layer it over several layers that you'd be wearing on the coldest outing. does it still fit? Not uncomfortably tight?
The last thing you want is to have to take of clothes on the coldest mornings because your down jacket is to tight.