Using EM's

calculators.scss

@function strip-units($number) {
  @return $number / ($number * 0 + 1);
}

@function em($target, $context: $base-font-size, $unitless: false) {
  @if ($unitless == false) {
    @return ($target / $context) * 1em;
  }
  @else {
    @return strip-units(($target / $context));
  }
}

example.scss

$base-font: 16px;
$card-title-font-size: 18px;
$card-meta-font-size: 13px;

body {
  font-size: $base-font;
}

.Card {
    font-size: em($card-title-font-size, $base-font);
}

.Card-meta {
    font-size: em($card-meta-font-size, $card-title-font-size);
    line-height: em(21px, $card-meta-font-size, false);
}