On this, there is currently no way to add a percentage size, because <$img> tags just get inserted into the text before export using the size tags. However, I have added an extra “ebook” tag for the next update. So, from now on, you could do this:
<$img:ImgName;w=400;ebook=50%>
In this example, the image will appear at 50% the width of the page on ebooks, but be 400pt across for all other formats. If you leave “ebook” out, then ebooks will use the same size as other formats. (“ebook” only works for ePub 3 and KF8, though.)