Made one with .67 argon/2.5 Clima/.67 argon to fit over my Marmot Hydrogen. I think it gives me about 15 degrees as an overquilt and I’m happy with it. I have only slept in in it by itself at a beach house in probably the low 60’s; but clealy it could go lower than that; for me I’d guess mid 50’s. I made mine with the footbox sewed closed up to about 36″ from the bottom, and for that distance it has no insulation for about 10″ wide, as I felt I didn’t need it there with a nice warm pad below. I used a tiny little buckle that had been on a pair of glove liners (to clip them together) at the back of the neck. sometimes I use the clip, sometimes it’s just draped. When I use the clip I don’t unclip it to get out – I just pull it over my head.
I worked out the size to fit over my bag without any compression so I get full loft on both, and long enough to get up over my chin. That does mean that it’s rather oversized for use on its own, but for the few times I am likely to do that it’s going to be real mild weather anyway so extra room is kinda nice. I suggest cutting everything out a little longer than you think, and then you can fine tune after you have all the layers together and can try it over the other quilt or bag.
I made a conscious decision to go for breathability over water repellency for the outer since my snow camping experience has always been that most nights I have more moisture on the inside of the shell of my bag than the outside. Had a bad pitch on my Duomid one night and ended up with fair amount of spindrift on the outside of the thing; no problem, and dried out real quick the next day.