a130/_sass/bourbon/functions/_flex-grid.scss

// Flexible grid
@function flex-grid($columns, $container-columns: $fg-max-columns) {
  $width: $columns * $fg-column + ($columns - 1) * $fg-gutter;
  $container-width: $container-columns * $fg-column + ($container-columns - 1) * $fg-gutter;
  @return percentage($width / $container-width);
}

// Flexible gutter
@function flex-gutter($container-columns: $fg-max-columns, $gutter: $fg-gutter) {
  $container-width: $container-columns * $fg-column + ($container-columns - 1) * $fg-gutter;
  @return percentage($gutter / $container-width);
}

// The $fg-column, $fg-gutter and $fg-max-columns variables must be defined in your base stylesheet to properly use the flex-grid function.
// This function takes the fluid grid equation (target / context = result) and uses columns to help define each.
//
// The calculation presumes that your column structure will be missing the last gutter:
//
//   -- column -- gutter -- column -- gutter -- column
//
//  $fg-column: 60px;             // Column Width
//  $fg-gutter: 25px;             // Gutter Width
//  $fg-max-columns: 12;          // Total Columns For Main Container
//
//  div {
//    width: flex-grid(4);        // returns (315px / 995px) = 31.65829%;
//    margin-left: flex-gutter(); // returns (25px / 995px) = 2.51256%;
//
//    p {
//      width: flex-grid(2, 4);  // returns (145px / 315px) = 46.031746%;
//      float: left;
//      margin: flex-gutter(4);  // returns (25px / 315px) = 7.936508%;
//    }
//
//    blockquote {
//      float: left;
//      width: flex-grid(2, 4); // returns (145px / 315px) = 46.031746%;
//    }
//  }