example.scss

$font-size-base: 16px;
$font-size-minor: 10px;

@mixin font-size($scale: 1) {
  font-size: round(
    ($font-size-base * ((exp(1.618, $scale) - exp(-0.618, $scale)) / 2.236)) + 
    ($font-size-minor * ((exp(1.618, ($scale - 1)) - exp(-0.618, ($scale - 1))) / 2.236))
  );
}