Refraction values should be 1 (white) or greater for natural materials (air is 1, glass is 1.33, diamond is 2.42). Right now the values you’re feeding refraction range only from 0 (black) to 1. So as a quick fix, between your final Multiply node and the Refraction pin use an “Add” node to add 1 to the result before it goes to the Refraction pin. This will boost everything up by 1 which I think will give you the effect you want (I’m unable to try it myself at the moment).
There’s more documentation on setting up refraction here. Although it doesn’t describe the falloff effect you’re creating, it might help to know these basics:
Let us know if my suggestion above works.