There are just too many variables in play to come up with a number that means anything.
Hydration/dehydration level, air humidity (huge factor), air movement, thickness of pad, sleeping surface temp and conductivity (rock, snow, dirt?) . . . finally, some people seem to sleep warmer than others in general and no one knows why.

You will sleep warmer if you've eaten, colder if you have not. The menu also affects nighttime metabolism, I believe . . . fats good, carbs not so much.

If you're physically exhausted you will sleep colder.

There is no standard system of measurement because of all these things. Sleeping bag manufacturers have to put a number on their bags to sell them, but the methods they use are all over the place. Some use water bottles, some use dummies heated to 98.6 degrees, some seem to guess. Kelty is wildly optimistic, Western Mountaineering is pessimistic. The two things manufacturers do in common are (as someone else here noted) assume you are in a tent and have a ground pad. So, that would seem to remove the ground pad and bivy from your calculations because they are already accounted for in the ratings.

You just have to play with it and tinker. Sleep in the yard in all sorts of weather until you come to some kind of comfort zone.

For me that arrangement would take me to zero degrees if it was a Western Mountaineering bag. With any other brand, all bets are off.