Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.4)
// Compass (v1.0.1)
// Susy (v2.1.3)
// ----

@import "susy";

@include with-layout(10 0 split){
  div.header{
    @include with-layout(12) {
      @include span(8 of 10) {
        .nested {
          @include span(3);
        }
      }
    }
  }
}

SassMeister-output.css

div.header {
  width: 80%;
  float: left;
}
div.header .nested {
  width: 37.5%;
  float: left;
}