flex.scss

@function flex($target, $context, $unit: "em") {
  $size: remove-unit($target) / remove-unit($context);

  @if $unit == "em" { @return #{$size}em; }
  @if $unit == "%" { @return perc($size); }
}

@function perc($target, $context) {
  @return flex($target, $context, "%");
}