Use the pixel MSE, not SSMI - I'm not really familiar with it, but it appears to be more about structure or perception. This is not what you care about, you want to see how much fine detail gets lost.
You'll have to play around with it, I would just try to Gaussian blur with different radii to see what works.
Alternatively (though roughly equivalent) you could try some kind of edge detection (and then summing the output) - especially if you're talking about blurred text, I'd expect there to be fewer hard edges in a blurry image
