Displaying a map at a shortcode

I needed to display the map in different places on various pages, so I created a shortcode tag to indicate where it is displayed. The code that sits in the theme’s functions.php is:

function pronamictag_func($atts) {
	extract(shortcode_atts(
		array(
			'width' => 500,
			'height' => 300,
		), $atts
	));

	return pronamic_google_maps(array(
		'width' => $width,
		'height' => $height,
		'echo' => false,
	));
}
add_shortcode('pronamic', 'pronamictag_func');

With this I can insert a shortcode tag anywhere in the content to display the map, like this:

[pronamic]

or

[pronamic width=600 height=200]

It would be nice if the width could be expressed as a percentage, but it is always interpreted as pixels.

— Jason