"If I wanted a quilt to take me to 30* F, with layers including down jacket and base layers, should I consider a 40* quilt or a 30*?"
If you have a good pad, and down pants,maybe booties, I would think 40 would work fine.
But it depends.
If you can shift down to feet and legs like in the revelation, and have a puffy jacket on, 40 might be fine too.
I was fairly cold once at 28 with my 40F EE quilt, NO overstuff., thin baselayer and thin fleece, +beanie, on a GG nightlight pad and 1/8" thin CCF pad. Lets just say I was glad when it was time to get up and move around in the morning. It was also 34 when I went to bed, so I was at freezing or below for about 9 hrs. I wrapped my rainjacket around my feet, and that might have made a 1-2F difference there, I could feel it.
My torso wasnt really cold. Only my legs and feet. So, with down jacket, down pants, down booties, and on a good pad, I bet I would have been cozy down to mid 20s.
On a multiday trip where the down will lose loft from absorbing some body moisture, I wouldnt intentionally plan to go below the rating of the quilt. The clothing may be needed for that situation after several days.