fill a gutter with padding from your el instead of margin.

fill-gutter.sass

// requires that you're using the susy grid system (you should be anyway.)

=span-columns-and-fill-gutter($cols, $context: false)
  @if ($context != false)
    width: space($cols, $context)
  @else
    width: space($cols)
  margin-right: 0
  padding-right: gutter()

// In use:

.my-el
  +span-columns-and-fill-gutter(1)

// Or, with context:

.my-el
  +span-columns-and-fill-gutter(1, 12)

Looks like you've mixed soft and hard tabs?

If you would rather keep the border-box model, you can add width: space($cols); You might also want to add an option for $context to be passed in so nested elements can use it. We may have to consider something like this in Susy Next. Care to file an issue?

@ericam I'm always fighting gist on tabs when I use the web interface.

I'll update this mixin and file the issue! Thanks for the feedback.