"If you have some .class { width:25% } that will make 4 columns grid (25% + 25% +25% + 25%). The problem is that you can't have precision px margins (gutter). So in the past you had nested divs or .something-like-this{width:24%; margin-left:1%}..."
"With calc() that problem is solved now we can have .class{width:calc(25%-10px); margin-left:10px} . And with that we can have the flexibility of the % and the px precision."
This was never a problem if you separated your layout markup from your content markup, like so.
<div style="float:left;width:50%">
<div style="margin:12px">Hello, world</div>
</div>
<div style="float:left;width:50%">
<div style="margin:12px">Hello, world</div>
</div>
Bear in mind that calc() has terrible support [http://caniuse.com/#search=calc]. Box-sizing fixes the problems addressed by this "framework" without deferring to an expensive and inappropriately used calc() property.I see no reason why anyone would use this over a grid model such as this [https://github.com/stubbornella/oocss/blob/master/core/grid/...].