I think one problem might be trying to solve this with the index value. If what you want is to select faces that point in a particular direction (down in this case,) it would be easier to use the face normal. With the Flip Faces node, you will be in face context (good), and the dot product with another direction vector will give you a scalar value you can use as a determinant.
This is a little more versatile. If you were to use a position input for the direction vector you could select faces pointing towards (or away) from the center.
An aside: Your node group was simple enough so instead of downloading your blend I built it myself using a toroid mesh (from the Extra Objects add-on I think?) The result was an index pattern that was completely different from yours so your node group didn’t work on my object. The normal/dot product solution should work on a toroid mesh, regardless of index pattern.
using new “set attribute” option in edit mode can be handy when you have difficulty selecting procedurally. You can create as many attributes as you like, boolean attribute for selecting, vector for rotation, float for scaling for example.