For example, I have a style at 12 points for a paragraph and need to place in it a couple of glyphs from a different font, but at 11 points. If I could say character style X is 96% of paragraph style Y, that would be great.

To my fortune every instance of my "different" 11 font has been surrounded by the same 12 paragraph, but if I had to use this elsewhere at a different size, I'd have to have another character style for that new size.