I just took the battle grid and made it 5ft cubes. I applied the same rules in the vertical plane.
See the attachment. One shows standard reach on the vertical and then what is normal for top down.
The second set shows the reach weapon having a ring outside of 5ft out to 10ft when you look at the top down view. But for the cubes in the vertical plane, I colored the 5 cubes on the ground level with 10ft elevation. Increase to 15ft elevation, the reach weapon no longer can attack targets on the ground that don't have large size (occupying cubes 10ft high).
In SnFA, Ausk is occupying a 10x10x10ft cube in my mind.
Does this all make sense?