dynamic variables for Sass (squaring the circle :-)

SassDynamicVariables.scss

@mixin makecircle($height: 10px, $width: 10px) {
    $radius: sqrt(($height * $width)/pi());
    @include border-radius($radius);
    height: $radius*2;
    width:  $radius*2;
}

.myCircle {
    @include makecircle(10px, 25px);   
}
// 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