shift($span)

$n: nth($span, 1)

$parent-n: parent-columns($span)
$expression: "(100% + #{$gutter}) * #{$n / $parent-n}"
$fallback: ($fallback-width - $gutter) * $n / $columns
+calc(margin-left, $expression, $fallback)

@each $breakpoint in $breakpoints
  +media($breakpoint)
    $parent-n: parent-columns($span)
    $expression: "(100% + #{$gutter}) * #{$n / $parent-n}"
    $fallback: ($fallback-width - $gutter) * $n / $columns
    +calc(margin-left, $expression, $fallback)