On my third or fourth quilt built now.
The 1st was a test out of cheap materials. Learned a lot of construction, but more after I bought a golite ultra 20 and saw how it was built.
2nd was a SUL summer M90 2.5 climashield. Too narrow. It was 48" at the top, 38" at the foot with a drawstring footbox. On that one I duplicated the golite ultra 20 drawcord setup which I like. Although okay in a hammock, too narrow on the ground. I turned it into a jacket.
Now since I am a side sleeper and my butt and knees stick out, I do this.
Sewn in footbox, if a drawstring add that to the length.
Sort of an hexagon shape. narrow top, wide body, tapered to a big footbox.
I like to cross and move my feet a lot.
This is what works for me....
Length of quilt = your height, IE for me 6'-3"
At the neck about 44" or so.
Widen out down about 14" from the neck to body circumference (around arms) which for me is 54"
Stays at 54" wide for 20" down the quilt. More width for knees and butt when side sleeping.
Tapers down to the footbox which is about 13.5" in Diameter.
Not SUL but it is really comfortable. Its 3.1 yards in area and a M50 shell weighs 5oz with a short zipper so I can get to the insides.
Here is a link to the pattern if you are interested. I made the neck smaller than this pattern.