Set Max Height as %

Is there a way to set a max height for an object as a percentage, instead of as a pixel amount? I’ve got a popup window that currently takes 90% of the browser height, but in larger resolutions, the content doesn’t take up the full 90%, more like 50%.

I want to be able to set an auto height for the object’s height, with a max height of 90%. Is there a way to do this?

I believe this is possible, you just have to type 90% in the max field and it should automatically convert it to a percentage value rather than a pixel value.