Watermark action : offset + relative width

I have a batch of original photos, huge sized. I want to watermark them to upload high-quality versions with a logo in the bottom-right corner, and also have a smaller sized batch for regular websites.
I made my actionlists, including the watermark feature with the offset option.
But some of my photos are cropped, so not every one has the exact same size. Thus the png watermark logo seems to be relatively bigger or smaller depending on the original picture size. A relative width option for the watermark, relative to global picture, on top of the offset option would be very nice.