Simple Sass helper for baseline grid value calculations.

baseline.sass

// set base font size if not set
// normalize.css standard value
$base-font-size: 16px !default

// define scope font size to make it available globally
$__scope: $base-font-size

@function relative($arguments)
  // ensure that the scope size is set
  $__scope: $base-font-size !default

  // prepare empty list for output
  $converted: ()

  // convert arguments to list
  $params: ($arguments)

  // iterate over arguments and convert
  // via scope if arguments is a pixel value
  @each $param in $params
    @if type-of($param) == 'number' and unit($param) == 'px'
      $converted: append($converted, em($param, $__scope))
    @else
      $converted: append($converted, $param)

  // return the converted list
  @return $converted

=with-font-size($value: $base-font-size, $base: $base-font-size)
  // set scope font size
  $__scope: $value

  // convert font-size relative to base font size
  font-size: em($value, $base)

  // yield to block
  @content

  // reset scope font size
  $__scope: $base-font-size

usage.sass

$base-font-size: 16px
 
@import "baseline"
 
#selector
  +with-font-size(20px)
    margin: relative(10px 20px)
    line-height: relative(20px)

// Output:
  #selector {
    font-size: 0.75em;
    margin: 0.83333em 1.66667em;
    line-height: 1.66667em; }

Nested example after adding the optional base argument:

#outer
  +with-font-size(10px)
    line-height: relative(15px)

  #inner
    +with-font-size(20px, 10px) // pass parent font size as base value
      line-height: relative(15px)

#outer {
  font-size: 0.625em;
  line-height: 1.5em; }

  #outer #inner {
    font-size: 2em;
    line-height: 0.75em; }