// helper function to make sure the unit type (px, rem) 
// is removed we want just the raw value no unit
@function get-value($n) {
    @return $n / ($n * 0 + 1);
}

// pretty simple eh? 
@function get-line-height($font-size, $line-height) {
   @return get-value($line-height / $font-size);
}

// Usage: 
// Let's say you have an element that is going to be
// 26px font-size and you want a line-height of 32px
// you don't want to set 32px as the height because 
// that value will cascade down to child elements
// but if you're like me you're too lazy to do 
// math all the time...

.foo {
    font-size: 26px;
    line-height: get-line-height(26px, 32px); //1.23077
}