dynamic variables for Sass (squaring the circle :-)

SassDynamicVariables.scss

@mixin makecircle($method) {
  // ? $height: getvalue(height).this;
  // ? $width: getvalue(width).this;
  @if ($method == area) {
    // using https://github.com/scottkellum/Sassy-math to do the math
    $radius: sqrt(($height * $width)/pi());
    @include border-radius($radius);
  }
    height: $radius*2;
    width:  $radius*2;
} .myCircle {
    height:10px;
    width:25px;
    // rectangle’s area = 250px²
    @include makecircle(area);
}
// should compile to:
.myCircle {
  height:17,8412411615277px;
  width:17,8412411615277px;
  border-radius:8,92062058076386px;
  // circle’s area = 250px²
}

// Okay, maybe I’m [en.wikipedia.org/wiki/Squaring_the_circle] here :-).
// But my real problem at hand is how to have Sass dynamically get the
// initial values for $height and $width, in order to have them recalculated
// by the mixin. Will this be possible in Sass?
// cfr: http://stackoverflow.com/questions/9394514/how-to-get-the-value-of-a-css-property-in-sass
// cfr: http://stackoverflow.com/questions/10377464/sass-get-value-of-existing-background-string-and-add-to-it