So I have been warm with 2.5 XP ( CLO .82 per oz), in a WB torso UQ, car camping between 46-49 and 95-98% humidity per nearby weather stations, right next to a lake, no warm base layers. ( https://www.hammockforums.net/forum/...ad.php?t=11090 ) though I was def warm and might could have got a few more degrees. But in order to have some slack built in for field conditions, I decided per this quote from the thread: "So, I think I can safely say that this ~9.5 oz synthetic torso quilt will be bombproof, all by itself, for 55 plus, pretty safe in the field for 50 or maybe even 45( especially with warm sleep clothes like fleece)." According to your formula, 2.5=55, and I said bombproof to 55. So I guess that is right close!
Cannibal has reported using 10 osy CS XP at a bit above zero ( in the snow with no tarp!), at minus 11F and some colder temp of maybe ~ minus 20F? And I think he was always warm enough, if mem serves(no doubt with plenty of base layers plus he sleeps real hot apparently). So maybe the minus 30.8F with 10 oz from your formula is really not all that far off at least for some people?
All of which I really find quite amazing, whether with 2.5 oz or 10 oz per sq.yd. I mean really, can anyone get much warmer with that amount of insulation, or a torso quilt of that total weight ( maybe 18 to 20 oz?). Maybe, a little, but that is still awfully good! CS is great stuff as long as you have room for it.