Proportional image resize

I'm having a little bit of a problem scaling my images to a properly predefined size. I was wondering - since it is purely mathematics, if there's some sort of common logical algorithm that works in every language (PHP, ActionScript, Javascript etc.) to scale images proportionally.

I'm using this at the moment:

var maxHeight   = 300;
var maxWidth    = 300;

var ratio:Number    =   height / width;

if (height > maxHeight) {
    height = maxHeight;
    width = Math.round(height / ratio);
} 

else if(width > maxWidth) {
    width = maxWidth;
    height = Math.round(width * ratio);
}

But it doesn't work properly. The images scales proportionately, sure enough, but the size isn't set at 300 (either in width or in height). It kind of makes sense, but I was wondering if there's a fool-proof, easy way to scale images proportionally.

ratio = MIN( maxWidth / width, maxHeight/ height );
width = ratio * width;
height = ratio * height;

Make sure all divides are floating-point.