span mixin using placeholder

SassMeister-input.scss

// ----
// Sass (v3.4.4)
// Compass (v1.0.1)
// ----

$grid-unit: 45px;
$grid-unit-mini: 15px;
$grid-unit-micro: 7px;

@mixin span($standard: 0, $mini: 0, $micro: 0, $extend: true) {
    $grid-units: (($standard * $grid-unit) + ($mini * $grid-unit-mini) + ($micro * $grid-unit-micro));

    @if $extend {

        @if $standard == 11 {
            @extend %span-11;
        }

        @if $standard == 12 {
            @extend %span-12;
        }
    }

    @else {
        width: $grid-units;
    }

}

%span-11 {
   @include span(11, 0, 0, false); // 495
}

%span-12 {
    @include span(12, 0, 0, false); // 540
}

.media {
  @include span(11);
}

.media-photo {
  @include span(12);
}

.media-video {
  @include span(11);
}

.media-slideshow {
  @include span(13, 0, 0, false);
}

SassMeister-output.css

.media, .media-video {
  width: 495px;
}

.media-photo {
  width: 540px;
}

.media-slideshow {
  width: 585px;
}