What rccardr said.

In my experience, you can have only so much spread between the 2 outer rings. The middle tooth count doesn't really matter. If you're going to run a 52 big ring, about 30 is the smallest you should go on the granny. So a 28 granny would mean a drop to a 50. Smaller than 28 means you'll need to run a big ring smaller than a 50.