Howto optimize culling in this example?

Hi Guys, can someone point out the best practice to optimize culling? See the current cull volume setting on the right, and the effect via game view.
Somehow there is always a gap between different LOD’s, same for the materials.

Should I use mesh cull settings too, or is there a recipe for the right cull volume settings? Or do I have to adjust mesh LOD settings as well?
Notice also that I use auto generated HLODs, but those doesn’t seem to affect the current structure culls.