Source of Zen theme variables This relates perhaps more to Sass then Drupal per se. But, it does come from a Drupal contrib theme; so- I'm hoping someone has encountered this or simply knows the answer. 

Question is: where are some of the Sass variable set. For example $nav-height

In the sass file _responsive.scss the following code 

/* Navigation bar */
@media all and (min-width: 480px) {
  #main {
    /* Move all the children of #main down to make room. */
    padding-top: $nav-height;
    position: relative;
  }
  #navigation {
    /* Move the navbar up inside #main's padding. */
    position: absolute;
    top: 0;
    height: $nav-height;
    width: $zen-grid-width;
  }
}


No where can I fine where $nav-height was initialized or set. Yet, when sass is compiled, magically - somehow, the value is there. 

Same question holds for the next variable "zen-grid-width".
$nav-height is specified in the file _init.scss (in the sass directory of your sub-theme) and the $zen-grid-width is set by the Zen Grids extension in the _grids.scss file (and if you're developing a non-responsive theme it is set in the _fixed.scss in the layouts directory of your sub-theme).