3-column fixed width Zen-based layout + SASS; giving exact width to columns
This is really more a math question than anything, but here goes.  Given that you're using a fixed width column layout, we have to start and find how many columns we need to achieve the proportions you posted. The least number of columns possible to achieve 220px | 480px | 220px is 46 with each column equating to 20px (920px / 46 = 20px). With 46 columns, the sidebars are 11 columns each and the main content is 24 columns.

The rest is just plugging in the numbers into the zen grids functions.  See http://vpaste.net/VDwL1 for my example.  

After defining the number of columns with $zen-column-count, you need to plug in the numbers so that each sidebar spans the correct number of columns while starting at the correct column with zen-grid-item($column-span, $column-position). The Zen starter sub-theme comes with three layouts to account for 1 (left) sidebar + main content, main content + 1 (right) sidebar, and 2 sidebars + main content. Here is how it looks with our new, 46 column system:

/**
 * The layout when there is only one sidebar, the left one.
 */
.sidebar-first {
  /* Span 35 columns, starting in 12th column from left. */
  #content {
    @include zen-grid-item(35, 12);
  }

  /* Span 11 columns, starting in 1st column from left. */
  .region-sidebar-first {
    @include zen-grid-item(11, 1);
  }
}

/**
 * The layout when there is only one sidebar, the right one.
 */
.sidebar-second {
  /* Span 35 columns, starting in 1st column from left. */
  #content {
    @include zen-grid-item(35, 1);
  }

  /* Span 11 columns, starting in 36th column from left. */
  .region-sidebar-second {
    @include zen-grid-item(11, 36);
  }
}

/**
 * The layout when there are two sidebars.
 */
.two-sidebars {
  /* Span 24 columns, starting in 12th column from left. */
  #content {
    @include zen-grid-item(24, 12);
  }

  /* Span 11 columns, starting in 1st column from left. */
  .region-sidebar-first {
    @include zen-grid-item(11, 1);
  }

  /* Span 11 columns, starting in 36th column from left. */
  .region-sidebar-second {
    @include zen-grid-item(11, 36);
  }
}