/// Divides `$value` by `$ratio` until it's below `$base`.
@function scale-below($value, $base, $ratio: 1.618) {
  @while $value > $base {
    $value: $value / $ratio;
  }
  @return $value;
}

$normal-font-size: 16px;
sup {
  font-size: scale-below(20px, 16px);
}

 

sup {
  font-size: 12.36094px;
}