Because dammit - you're a designer, not a mathematician.
Susy is a grid system for web designers. Built on the techniques of Natalie Downe, the Compass authoring framework, and the Sass meta-language for CSS, Susy brings together the unique semantic control of hand-written code with the speed of a framework and the strength needed for a bullet-proof design. With Susy you can build quick, custom grids that respond to the needs of the user without giving up design integrity. It's elastic, it's fluid, it's Susy.
Susy is an open source project envisioned and maintained by the OddBird Collective: Eric Meyer, Carl Meyer and Jonny Gerig Meyer, all of whom would love to chat sometime. Give them a ring.