What opacity of Cool Gray 11 gives you Cool Gray 9?

I assume that you want an object that is transparent against a background that will give the cool grey 11 the feeling that it is in fact cool grey 9. I found that a percentage of 78% of the cool grey 11 against a white background using the swatches from the PANTONE Color Bridge Library will simulate cool grey 11. However, since the transparency of the color will be totally influenced by the colors that it is put in front of it makes me wonder what your real purpose is.