Speaking from my own experience, I'm not sure it makes much difference what your quilt is rated when you are using a quilt in warmer temperatures. What I mean is a 0 or 10 or 20 deg quilt will feel about the same as a 40 deg quilt if you are using it on a 60 deg night, even a 0 deg quilt will not be uncomfortably warm. It only makes a difference when it's cold out and you need the insulation to maintain body heat.
My experience is based on using my 0 deg quilts year round (actually a 0 deg under quilt and a 20 deg top quilt supplemented with a mummy sleeping bag when needed). Last October, when it started getting cooler, I decided to start sleeping outside every night for a while. After a couple of weeks I was really starting to enjoy being outdoors every night. After a month outdoors I wondered if I could go a whole two months straight. I have an open, but covered deck, so rain was no problem, though there were a few nights when I had to put up a tarp due to wind blown rain. After two months I wondered how cold it could get before I bugged out and went back to sleeping inside. The coldest it got here in Nashville last winter was 12 deg, and I stayed snug as a bug that night. And that coldest night was at the end of January. So, at 4 months in I thought why not try going six months. (You might say I was becoming obsessed!)
I was relatively certain I wouldn't be able to handle the hot and humid summer nights we have here in the South, so I figured 6 months would be it. But nope, on October 15 I will have slept outside in a hammock every night for a year. And I've used a 0 deg under quilt all but about 4 or 5 nights this summer, and by the morning time I was wishing the quilt was under me a couple of those nights.
I'm sure my body has just become acclimated to being outside every night so the temperature and humidity don't bother me so much. But the point I am trying to make is, unless pack weight is an issue, you probably only need one quilt for any temperature. Make it a quilt rated for the coldest (and then subtract 10 deg) you plan to go.
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