Bushwalk Australia

From a link to BPL
X -(70 - Y)/2 = Z
In Fahrenheit but it works in a Celsius conversion also
Where X is the Outer/warmer bag and Y in the Inner/cooler bag
I've just used it for my combination and I think it gives a very conservative rating so suitable for cold sleepers such as myself
I used it to work out the limit for my own combination of the half bag and parka inside my Ultra Flouro and got a rating of -20C for my legs and -30C for my torso
Which is what I have been assuming for ages as the top half but I was a little surprised at the leg rating as it "feels" a lot warmer than that

I suspect you get a better result than the formula suggests because you probably have a lot less warm air escaping from that inner bag when moving than if you were using the inner bag alone in its intended temperature range.
When I use my Ultralite at -5/7c something I am warm but I notice more of a temperature change when I roll inside that one (just with an underlayer) than when I do the same inside the Summerlite with puffy clothing.
possibly just less air space does the trick (the Ultralite and Summerlite have about the same inner dimensions)
BTW, I have suggested many times that theory and practice don't always correspond.

Just adding thickness of insulation would indicate a somewhat lower temperature rating as total loft exceeds 200mm in the legs and close to 270mm in the torso

PHD reckon their -6C rated Hispar Combi bag with their -9C Hispar 400 is good to -33C, and the Combi+Hispar 500 is rated to -39. This formula gives -21C and -24C. Pretty big difference, although PHD's ratings are suspect. Also not sure why the outer bag has to be the warmer one, or why it would make a big difference. If you do it the other way around you get -22.5 and -28.5 which is closer.

Specifically for me a -7 and -10 EE Revelation this would give a value of -23.5C (or -26C). Which seems a bit high given the amounts of down involved (about 975g of 800 in the Revelations) but in the right ball park (a WM Lynx has 965g of 850, but a lot of that is on the bottom).

I'd also take this formula with a huge grain of salt, but it provides another data point at least.

Well PHD will not put any of their bags to the EU test so I have always assumed it's a "Best case" rating and not at all a comfort rating
It is a formula to give us a ball park figure tho.