hummmmm …. I can take that in a couple of different ways ;o)
Seriously …. I have a shoulder girth of 60 inches, give or take a half inch when measured around the biceps.
I'm about 5'11" tall and I toss and turn A LOT during the night.
I've found that adding 10 inches to your max girth is a good rule of thumb for max comfort for the sleep system. You can get by with 8 inches, but 10 works best.
So …. a 20 inch wide pad, subtracted from a 60 plus 10, or 70 inch requirement, leaves you with about 50 inches. Now …. how much material do you want to drape on the groud and overlap? That's the real question.
54 inch quilts leave me drafty when I turn over, so I would use less width for a warmer weather quilt and more for a colder weather quilt. For me, assuming my quilt will go down to 30, I like a 58 inch width, leaving me 8 inches of overlap, 4 per side, that I can tuck under the pad, or roll up in, or whatever.
straps also change the equation, as long as you're always planning on using them, and reduces the amount of overlap you need …. any way you look at it, I wouldn't feel comfortable for a cool weather quilt to go less than a 56 inch width with straps.
For a summer quilt, However, I have a 48 by 72 inch quilt that comes up to my armpits. I've used drawstrings in the foot area to make a sortof footbox that works well enough for 45 and above, then used one layer of primaloft(.6 inch loft). This is more than enough with an insulated jacket or vest on and a sock hat for warmer temps.
That quilt comes in at about 11 oz but has a higher clo than xp (.83).
As for the thickness of the insulation … well, that's very dependent on you.
My formula is 60 – (total clo * comfort factor)= temp rating.
You can figure out your current comfort factor by looking at what you currently use for a sleeping bag and calculating backwards. My comfort factor is about 7.9 … so, XP with a total clo of around 2 will yield a temp rating for me of about 42 degrees all alone. Clothing or a jacket will adjust this.
My wife, however, would freeze with that same quilt. Her comfort factor is more in the range of 5. The same quilt would be a 50 degree quilt for her.
How warm or cold you sleep will be the difference in what your personal comfort factor happens to be. I happened to put a double layer of xp in the foot box of my quilt because if my feet and torso are warm I can still sleep if my legs are chilled. Once my torso, head, or feet get cold I'm done.