This means the sum of top space and bottom space is going to be:
total-space = font-size*line-height - font-size = (line-height - 1)*font-size = (line-height - 1)*1em
Note we have been using 1em as font-size; this is because the font-size of a ::before pseudo-element is, by default, equal to the font-size of the element it is attached to.
Assuming bottom and top spaces are equal, it gives you:
top-space = (line-height - 1)*1em/2 = (line-height - 1)*0.5em
The opposite of this top-space value gives you the value used in the mixin (you want a negative margin to crop the top space).
We could have created a similar mixin for the bottom space (or included it in the lhCrop mixin) but this is something we have not been needing so far (so we probably won’t be including it).
... read the whole story at medium.com.