$z-elements: 'main'
             'header'
             'overlay-above-header';

$z-layers: ();
$z-multiplier: 10;

@each $z-element in $z-elements {
  $z-index: $z-multiplier * index($z-elements, $z-element);
  $z-layers: map-merge($z-layers, ($z-element: $z-index));
}

@function z($layer) {
  @if not map-has-key($z-layers, $layer) {
    @error "No layer found for `#{$layer}` in $z-layers map. Property omitted.";
  }
 
  @return map-get($z-layers, $layer);
}