A solution for handling of z-indexes in complex apps.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

$layers:
    '-1|underlay',
    'base',
    'header',
    '999|an-overlay',
    'another-overlay'
  ;

/*
  A mixin that provides calculated z-index value of a given layer
  (Layer must be predefined in $layers SCSS list).
  
  By using this mixin you don't explicitly define your z-index values in declaration blocks 
  but just add it as a layer here, so you can have a full control over your z-indexes in one place.
*/
@mixin z-index($layer, $addition: 0) {
  $START_FROM: 0;
  $INCREMENT_BY: 1;
  
  $zIndex: -1;
  $customizedLayers: 1;
  @for $index from 1 through length($layers) {
    $currentItem: nth($layers, $index);
    $currentLayer: $currentItem;
    
    @if $index - $customizedLayers == 0 {
      $zIndex: $START_FROM;
    } @else {
      $zIndex: max($zIndex + 1, $index - $customizedLayers) + $INCREMENT_BY - 1;
    }
    
    $separatorIndex: str-index($currentLayer, '|');
    @if $separatorIndex {
      $customizedLayers: $customizedLayers + 1;
      $currentLayer: str-slice($currentItem, $separatorIndex + 1, str-length($currentItem));
      $fixedZIndex: str-slice($currentItem, 0, $separatorIndex - 1);
      $zIndex: to-number($fixedZIndex);
    }
    
    @if $currentLayer == $layer {
      z-index: $zIndex + $addition;
    }
  }
}

@function to-number($value) {
  /*
    A helper function that casts a string into a number.
  
    http://hugogiraudel.com/2014/01/15/sass-string-to-number/
    http://sassmeister.com/gist/9fa19d254864f33d4a80
  */
  @if type-of($value) == 'number' {
    @return $value;
  } @else if type-of($value) != 'string' {
    $_: log('Value for `to-number` should be a number or a string.');
  }
  
  $result: 0;
  $digits: 0;
  $minus: str-slice($value, 1, 1) == '-';
  $numbers: ('0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9);
  
  @for $i from if($minus, 2, 1) through str-length($value) {
    $character: str-slice($value, $i, $i);
    
    @if not (index(map-keys($numbers), $character) or $character == '.') {
      @return to-length(if($minus, -$result, $result), str-slice($value, $i))
    }
    
    @if $character == '.' {
      $digits: 1; 
    } @else if $digits == 0 {
      $result: $result * 10 + map-get($numbers, $character);  
    } @else {
      $digits: $digits * 10;
      $result: $result + map-get($numbers, $character) / $digits;
    }
  }
  
  @return if($minus, -$result, $result);
}

#underlay {
  @include z-index('underlay');
}

.content {
  @include z-index('base');
}

.header {
  @include z-index('header');
}

#an-overlay {
  @include z-index('an-overlay');
}

.another-overlay {
  @include z-index('another-overlay');
}

SassMeister-output.css

/*
  A mixin that provides calculated z-index value of a given layer
  (Layer must be predefined in $layers SCSS list).
  
  By using this mixin you don't explicitly define your z-index values in declaration blocks 
  but just add it as a layer here, so you can have a full control over your z-indexes in one place.
*/
#underlay {
  z-index: -1;
}

.content {
  z-index: 0;
}

.header {
  z-index: 1;
}

#an-overlay {
  z-index: 999;
}

.another-overlay {
  z-index: 1000;
}