a.rb

@mixin font-size($percent, $min-font-size, $max-font-size) {
    // Min
    font-size: $min-font-size;

    @include breakpoint($min-font-size/$percent * 100) {
        font-size: #{$percent}vw;
    }

    @include breakpoint($max-font-size/$percent * 100) {
        font-size: $max-font-size;
    }
}