I have thought about this in far too much length. Obviously down rating and down fill can be compared but then the cut of the jacket comes into play. What do the baffles look like? Even the fabric comes into play - will you loose heat via convection?

Some manufacturers do post an expected comfort range. I have no idea if they are performing EN testing (can you?).

I have a few down jackets. My Ghost Whisperer is sometimes warm at say 10C and sometimes it is cold. To an earlier point, sometimes I am tired and less well fed and the jacket doesn't keep me warm.