How can I add an image overlay with a relative size?

When adding overlays, there are the following options:

Specifying the overlay size in absolute pixels. For example, 100px height:

overlay1

Specifying the overlay in percents of the original overlay size. For example, 80% of the original overlay's height:

overlay2

Specifying the overlay size in percents of the containing image. For example, 80% of the containing images height (200px in this case)

overlay3