Setting a length (height or width) for one element minus the variable length of another, i.e. calc(x - y), where y is unknown

Question

I know we can use calc when lengths are defined:

flex-basis: calc(33.33% - 60px);
left: calc(50% - 25px);
height: calc(100em/5);

But what i

Answer 1

IN CSS

Although I've never tried it, I believe that this would work:

.top {
    height:13px;
}

.main {
    height:calc(100% - var(height));
}

http://www.creativebloq.com/netmag/why-you-need-use-css-variables-91412904

---

IN SASS

$top_height: 50px

.main {
    height: calc(100% - $top_height)
}

Sass Variable in CSS calc() function