$z-index: (
  modal: 100,
  modalBG: 90,
  navFixed: 80,
  tooltip: 70,
  popover: 60,
  dropdown: 50,
  nav: 50,
  contentTop: 40,
  content: 30,
  contentBottom: 20,
);

// useage:
// .class { z-index: z("modal"); }
@function z($key) {
  @return map-get($z-index, $key)
}