Make breakpoint configurable.
Remove unused variable $i.
Output all shared properties only once.
Collapse calculations.
More math, less variables. @for instead of @while.

Cool: the output CSS is now ordered ascending.
Long variable names.

More explicit than abbreviated variables.
`scss-lint`.

1. Line should be indented 2 spaces.
2. Name of variable should be written in lowercase with hyphens.
3. Prefer single quoted strings.
4. Rule declaration should be followed by an empty line.
5. Properties should be sorted in order.
6. Files should end with a trailing newline.