New CSS grid at 1800px
I want to suggest that you use LESS/SASS to build your library 

You have a lot of math. One problem here is that if you want to change your sizes, you'd be facing a calculator, calculate and change values for each. One decimal place wrong and it could ruin your pixel-perfect calculations. 

With LESS/SASS, you can define your values in variables, and have properties as math expressions. Want to change the dimensions? No need to meddle with the implementation, just change the variables (That's how BootStrap customizers work).

@fullWidth : 1800;

.fifty {
  width : percentage(@fullWidth/2);
}


Also, it has nesting. I think this is possible in place of your very long sequence up there. Much cleaner.

.belt {
  .m1, .m2, .m3, .m4, .m5, .m6, .m7, .m8 {
    ...
  }
}