Gradient Fade on Passthrough Surface

Hello! 

I'm working on a MR experience and would like to apply a passthrough surface to the walls of my room. On this passthrough surface Id like to gradually fade the passthrough effect so that the ceiling and top portions of each wall show digital content and the lower half of the room displays the physical space.

Right now Im using a stencil shader attached to the ceiling to display content which you can see in the photo below. 

Basically instead of the hard edges on the stencil mask attached to the ceiling  I want it to gradually fade into passthrough. Im pretty new to creating custom shaders so any advice on this would be greatly appreciated!! Thanks in advance.