I tried your formula, and it doesn't work for my experience.
So I though I should share
25+ years ago I lived in a tent through a couple of winters that had periods of extreme cold eg. -40
I was using two mummy/Dacron hollow fill bags that were rated to -7c
The bags were twins.
your formula would suggest:
-7c or 19.4 degree Fahrenheit
70 - 19.4 = 50.6F/2 = 25.3F
-19.4 - 25.3 = -5.9F
-5.9 degree Fahrenheit = -21.0555556 degree Celsius
So the combo of the two bags is good to -21c according to the formula.
I guess the question is to what degree of comfort is that -21c.
Here is my experience:
In my tent I slept in the two bags, on a single mattress off the ground.
I slept in boxer shorts and Tee shirt only . . .
(When it got colder I should have worn long johns. Duh.)
. . . Down to -34c/-29.2F I was just OK
-36c/-32.8F and I was not wanting to even wiggle my toes as they would feel the extreme cold of the bag.
-38c/-36.4F and I would spend 1/4 of the night, I guess closer to morn, shivering.
-40c/-40F I just happened to be far away from home, and in a motel room
- I was checking out a wood lot to horse log.
I never got a chance to do it, but that is where I was.
One thing is for sure, in the morning, minus the wind,
temp inside and out of the tent are equil.
More info: the tent had a 3/8" plywood floor on 2x6 joist,
one corner on the ground and the other three some what elevated,
it was built on a small slope.
The tent walls were a light industrial tarping, no insulation value.
The wood cook stove with oven, heated well enough to sponge bathe late in the evening.
Lighting was an old coal oil table lamp.
Shortly after bedding down the fire was out.
In the dead of winter the sun was down at 16:30,
and I would only spend an hour or two with supper and a night cap, journal entry if the pen worked.
I don't know if that helps, but it is just as it happened to me.