Font Offset

font-helpers.scss

$font-adjustments: (
    arial-bold: (
         ascender:  0.056,
              cap: -0.130,
         baseline: -0.154,
        descender:  0.056
    ),
    georgia-regular: (
         ascender:  0.064,
              cap: -0.157,
         baseline: -0.149,
        descender:  0.064
    ),
    futura-light: (
         ascender: -0.016,
              cap: -0.202,
         baseline: -0.130,
        descender:  0.098
    ),
    futura-book: (
         ascender:  0.042,
              cap: -0.172,
         baseline: -0.160,
        descender:  0.070
    ),
    futura-bold: (
         ascender:  0.052,
              cap: -0.204,
         baseline: -0.128,
        descender:  0.100
    ),
    source-sans-light: (
         ascender: -0.038,
              cap: -0.194,
         baseline: -0.146,
        descender:  0.082
    ),
    source-sans-regular: (
         ascender: -0.036,
              cap: -0.198,
         baseline: -0.146,
        descender:  0.070
    ),
    source-sans-semibold: (
         ascender: -0.034,
              cap: -0.200,
         baseline: -0.146,
        descender:  0.060
    )
);

@mixin font($family, $size, $height: normal) {
    @extend %#{$family};
    font-size: $size;
    @if type-of($height) == number and not unitless($height) {
        line-height: ($height / $size);
    } @else {
        line-height: $height;
    }
}

@function font-offset($font, $trim-to, $size, $line-height) {
    $adjustment: map-get(map-get($font-adjustments, $font), $trim-to);
    @if (unitless($line-height)) {
        $line-height: $size * $line-height;
    }
    $neg-half-leading: ($line-height - $size) / -2;
    @return $neg-half-leading + $size * $adjustment;
}

foo.scss

.foo {
    $font-size: 14px;
    $line-height: 22px;
    @include font(source-sans-regular, $font-size, $line-height);
    margin-top: font-offset(
        $font: source-sans-regular,
        $trim-to: cap,
        $size: $font-size,
        $line-height: $line-height
    );
    margin-bottom: font-offset(
        $font: source-sans-regular,
        $trim-to: baseline,
        $size: $font-size,
        $line-height: $line-height
    );
}