I'm attempting to make a simple shader whereby an object has a filling effect using an alpha cutoff. What I'm looking to do is to fill in the "top" of the alpha cutoff within the object as it goes up and down, avoiding the transparent back faces.
I've attached a gif of the shader and a shot of the node setup. I've turned Cull Mode to Off for the moment which works as the object is emissive making the missing faces not noticeable, however, I'm just wondering is there a method to do so as mentioned above?
Thanks for your help.

https://i.imgur.com/CqYuzmZ.gifv