How to set the background to an incomplete width and at the same time cover?

Question

css

It's time for stupid questions.
It is necessary to give the block of the first screen a background image that will always occupy 80% of the width and 100% of the height. At the same time, to be, as it were, Сover, that is, not to be distorted. With an img image and a wrapper for it, no problem, but with a clean background, the section does not work, I already read the specs, my eyes are blurred somewhere, I ask for tips.
I get similar behavior with the rules

background-size: cover;
background-position: 100px top;

but if you replace the pixels with percentages, then it stops working

Answer 1