CSS, Sass: Flexible media query mixin

flexible-media-query-mixin.scss

//base font
$bf: 16;

@mixin bp($bp, $min-max: min-width) {
    $em: $bp/$bf;
    @media(#{$min-max}: #{$em}em) { @content; }
}

//including the mixin
.element {
    @include bp(600) {
        ...
    }
}