Using Two Sleeping Bags / Quilts for Winter Camping

Can I combine my sleeping bag and my quilt to get a good winter sleeping bag? I have a synthetic, big, heavy winter sleeping bag, an ultralight 3-season down quilt and an old down summer sleeping bag. How warm is my quilt and my summer sleeping bag together? Here is some math I've found on the internet:
For Celsius
Comfort temperature from one bag (10) + comfort temperature from the other bag (0) - most comfortable temperature (25) +7 = the new temperature raiting
10+0-25+7=-8°C
For Fahrenheit
Comfort temperature of the warmer sleeping bag - (70°F- comfort temperature of the colder bag)/2 = new temperature raiting
With my quilt and sleeping bag:
32-(70-50)/2=
32-20/2=
32-10=22°F
Don't trust the formula completely! It will be only approximately correct. Even with the two formulas above I get results with a difference of about 2.5°C or 4.4°F.
One thing I know for sure when I've used the sleeping bag combination in -5°C/23°F. I had a good and warm sleep, which I wouldn't get with only the quilt. I don't know where these formulas came from. I've found them on a few different websites.
Sleeping bags, quilt and other equipment in this video:
Thermarest Vesper Quilt -6C/20F
Mountain Hardwear Lamina -18
Some sort of summer sleeping bag
Therm-a-Rest Neoair Xtherm

[@aksting]

It would be better to keep the quilt on the top. This way the loft of the down is not lost. It is pretty easy to compress the quilt inside of a sealed sleeping bag.

[@koenkerkhoff1781]

Goed filmpje wel!
Hoppa abbo gedaan

[@shempdoll]

I think everyone sleeps so differently it is difficult to know how your bag combination will work until you try it. My approach is opposite of yours, i try to get as cold as possible before zipping up the bag. Also important to be as dry as possible with fresh under clothes and prevent sweating during the night. Also keeping an empty bottle in the tent is handy for all of the hot drinks consumed in the evening.

[@zaheraldik5016]

I am active sleeper I turned and toss around a lot, I hate mummy sleeping bag, I am thinking of doing set up similar to yours ,where I get rectangular sleeping bag along with down quilt .

[@ballebart]

Interesting formulas. What exactly is meant with "most comfortable temperature"?
In case you're not in the opportunity to make a campfire to warm up before going to bed, an aluminium water bottle can come in handy: use it to store drinking water for during the day and use it as a hot water bottle during the night. (Assuming a little stove is part of your're equipment.)

[@MrOutdoorAdventure]

With most comfortable temperature I mean a temperature where I don't get cold without moving and I don't start to feel lazy because it's too warm. I hope that describes it well. But I would just take 25°C for this formula. I think than it works well as long as the sleeping bags are not getting compressed and can work efficiently.
Yes, I like to use a hot bottle in the sleeping bag as well. Also, the drinking water isn't frozen in the morning.

[@boscho1986]

To derive a formula you can think as follows: your inner sleeping bag of the two will still keep you warm, when the temperature on it's outside, i.e. the temperature in between the two bags, is not lower than it's comfort-temperature - e.g. 10C. Now you only have to know, how cold it can be outside the outer bag so it is not getting colder than 10 on it's inside. You know, that you can sleep well in the outer bag alone, if it is not colder than it's comfort-temperature - say 8C. So the outer bag is capable of keeping you warm (i.e. at 25C) when it is not colder than 8C on it's outside, i.e. it "bridges" a temperature gap of 25C - 8C = 17C. If it has to be 10C in between you inner and your outer bag to sleep well, than the outside temparature can be as low as 10C - 17C = -7C. Of course this only holds, if these bags fit neatly inside of each other: if their fillings are compressed, their combination will be colder. Also be aware, that estimating your next-to-skin temperature to high gives you a to warm rating for your combination. This is so, because a to high value for your next-to-skin temperature in this calculation, will result in a to high temperature gap this bag supposedly is capable of filling: e.g. we had 25C as next-to-skin temp and got 25C - 8C = 17C as possible gap. If you assume your next-to skin temp is not 25C but only 21C, you would get 21C - 8C = 13C as capability and the combination would end up with 10C - 13C = -3C. I usually calculate with 20C, which is probably to low an thus giving me some reserve in my calculations. Happy camping!

[@stevejedeli]

Der Zeitpunkt für das Video könnte nicht besser sein! :D Ich hab' mir erst gestern den Vesper Quilt -6C/20F bestellt. Ich freu' mich schon den zu testen bei ähnlichen Temperaturen. Zufällig liegt bei mir auch noch ein ungenutzter Decathlon Sommer-Schlafsack rum. Den werde ich dann genau so verwenden, wie du im Video vorschlägst. 👌Mal wieder super gefilmt und vertont das ganze!

[@MrOutdoorAdventure]

Vielen Dank für den netten Kommentar und viel Spass auf der ersten Tour mit dem neuen Quilt. Ich bin mit meinem bis jetzt vollkommen zufrieden. Vor allem in Kombination mit einer Hängematte finde ich den Quilt viel angenehmer.

[@yourloved6741]

Is that xthern sleeping pad? I just ordered a reg wide. Do you find the Reg works ok?

[@waynestewart3232]

You add seven for science.

[@gen81465]

Because Neil DeGrasse Tyson said so!