Experiment using Sass maps to organize Susy Next configuration. Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.10)
// ----

$grid: (
  small:                  90%  8 1/4,
  medium:                 80%  8 1/4,
  large:                  80% 12 1/4,
);

$content-grid: (
  left:                   100% (4 8) 1/4,
  right:                  100% (8 4) 1/4
);

@function _grid($map, $g, $attr: false) {
  $list: map-get($map, $g);
  @if $attr == container-width { @return nth($list, 1) }
  @if $attr == columns         { @return nth($list, 2) }
  @if $attr == gutters         { @return nth($list, 3) }
  @if $attr == total-columns {
    $columns: nth($list, 2);
    @if type-of($columns) != list { @return $columns }
    $sum: 0;
    @each $n in $columns {
      $sum: $sum + $n;
    }
    @return $sum;
  }
  @return $list;
}

@function grid($g, $attr: false) {
  @return _grid($grid, $g, $attr);
}

@function content-grid($g, $attr: false) {
  @return _grid($content-grid, $g, $attr);
}

output {
  a: length(map-get($grid, small));
  b: grid(small);
  c: grid(small, container-width);
  d: content-grid(left, gutters);
  e: grid(medium, columns);
  f: grid(medium, total-columns);
  g: content-grid(left, total-columns);
}

SassMeister-output.css

output {
  a: 3;
  b: 90% 8 1/4;
  c: 90%;
  d: 1/4;
  e: 8;
  f: 8;
  g: 12;
}

So if Susy used this pattern, my example grid() function above would correspond to susy-get().

Question: does susy-get() return the total number of columns in a grid? Not sure the best way to phrase this, but for instance, a grid with columns: (4 8) has a total of 12 columns. I think that would be a useful feature.