Removed Singular and Halfs, as well as Quarters.

input.scss

$cols: 12;
$max-width: 960;
$viewport-namespace: "desk";
$gutter-width: 20;
 
@while $cols > 2 {
  @for $i from 1 through $cols {
    @if $cols != 4 {
      .#{$viewport-namespace}-#{$i}-#{$cols} {
        $col-width: ($max-width / $cols * $i - $gutter-width);
        width: (percentage($col-width / $max-width));
      } 
    }
  }
  $cols: $cols - 1;
}

output.css

.desk-1-12 {
  width: 6.25%;
}

.desk-2-12 {
  width: 14.58333%;
}

.desk-3-12 {
  width: 22.91667%;
}

.desk-4-12 {
  width: 31.25%;
}

.desk-5-12 {
  width: 39.58333%;
}

.desk-6-12 {
  width: 47.91667%;
}

.desk-7-12 {
  width: 56.25%;
}

.desk-8-12 {
  width: 64.58333%;
}

.desk-9-12 {
  width: 72.91667%;
}

.desk-10-12 {
  width: 81.25%;
}

.desk-11-12 {
  width: 89.58333%;
}

.desk-12-12 {
  width: 97.91667%;
}

.desk-1-11 {
  width: 7.00758%;
}

.desk-2-11 {
  width: 16.09848%;
}

.desk-3-11 {
  width: 25.18939%;
}

.desk-4-11 {
  width: 34.2803%;
}

.desk-5-11 {
  width: 43.37121%;
}

.desk-6-11 {
  width: 52.46212%;
}

.desk-7-11 {
  width: 61.55303%;
}

.desk-8-11 {
  width: 70.64394%;
}

.desk-9-11 {
  width: 79.73485%;
}

.desk-10-11 {
  width: 88.82576%;
}

.desk-11-11 {
  width: 97.91667%;
}

.desk-1-10 {
  width: 7.91667%;
}

.desk-2-10 {
  width: 17.91667%;
}

.desk-3-10 {
  width: 27.91667%;
}

.desk-4-10 {
  width: 37.91667%;
}

.desk-5-10 {
  width: 47.91667%;
}

.desk-6-10 {
  width: 57.91667%;
}

.desk-7-10 {
  width: 67.91667%;
}

.desk-8-10 {
  width: 77.91667%;
}

.desk-9-10 {
  width: 87.91667%;
}

.desk-10-10 {
  width: 97.91667%;
}

.desk-1-9 {
  width: 9.02778%;
}

.desk-2-9 {
  width: 20.13889%;
}

.desk-3-9 {
  width: 31.25%;
}

.desk-4-9 {
  width: 42.36111%;
}

.desk-5-9 {
  width: 53.47222%;
}

.desk-6-9 {
  width: 64.58333%;
}

.desk-7-9 {
  width: 75.69444%;
}

.desk-8-9 {
  width: 86.80556%;
}

.desk-9-9 {
  width: 97.91667%;
}

.desk-1-8 {
  width: 10.41667%;
}

.desk-2-8 {
  width: 22.91667%;
}

.desk-3-8 {
  width: 35.41667%;
}

.desk-4-8 {
  width: 47.91667%;
}

.desk-5-8 {
  width: 60.41667%;
}

.desk-6-8 {
  width: 72.91667%;
}

.desk-7-8 {
  width: 85.41667%;
}

.desk-8-8 {
  width: 97.91667%;
}

.desk-1-7 {
  width: 12.20238%;
}

.desk-2-7 {
  width: 26.4881%;
}

.desk-3-7 {
  width: 40.77381%;
}

.desk-4-7 {
  width: 55.05952%;
}

.desk-5-7 {
  width: 69.34524%;
}

.desk-6-7 {
  width: 83.63095%;
}

.desk-7-7 {
  width: 97.91667%;
}

.desk-1-6 {
  width: 14.58333%;
}

.desk-2-6 {
  width: 31.25%;
}

.desk-3-6 {
  width: 47.91667%;
}

.desk-4-6 {
  width: 64.58333%;
}

.desk-5-6 {
  width: 81.25%;
}

.desk-6-6 {
  width: 97.91667%;
}

.desk-1-5 {
  width: 17.91667%;
}

.desk-2-5 {
  width: 37.91667%;
}

.desk-3-5 {
  width: 57.91667%;
}

.desk-4-5 {
  width: 77.91667%;
}

.desk-5-5 {
  width: 97.91667%;
}

.desk-1-3 {
  width: 31.25%;
}

.desk-2-3 {
  width: 64.58333%;
}

.desk-3-3 {
  width: 97.91667%;
}