I found the useful website, which is here:

https://stjarnhimlen.../radfaq.html#11

and we can find there the values like this:

and this:

most of them like "Overcast sky" or "Twilight" and especially the lower table refers to the situation, when the solar apparent magnitude is -26.77.

What I am after is the same computation for the situation if our primary light source would have i.e. -25Mag, -24,5Mag, -22Mag and so forth.

I found some hint here

What's the conversion between apparent magnitude and lux?

and did some computations, however they refer only to the source of light itself. I am still unable to estimate the illumination for the twilight sky when the Sun is i.e. 6 degrees below the horizon, etc.

Moreover I would be interested in calculations of the sky surface brightness in various directions based on the **stellar magnitude of the light source** and its **position against the horizon**. Is there any (quite simple) formula? Which would calculate this stuff? In Stellarium when we click on the Sun, we have its apparent magnitude and the magnitude "reduced by airmasses". I would need the formula for it either if possible.

Any hint will be cordially appreciated!