To be clear: now that we have border-box sizing, grid math can be much simpler. Complex math is only required if you are using margin-based gutters. Move your gutters into padding, and the math is simple. That’s why I don’t use Susy (or any grid system) much any more.
percentage(1/5)works great in Sass - and flexbox takes care of the rest.
You can read a summary of the conversation as well – but it leaves out some good bits. ;)