Need some scope direction in front of thermal clip on.

[Laser1]

I'm looking for a scope to go behind a clip on thermal, probably a Steiner clip on. I had read that it needed a ring height of 1.5" to properly co-witness. I'm looking to get a scope 2 x 15, 2 x 18 maybe 2 x 20. My question is: What is the max objective diameter I can get away with to properly clear the rail while maintaining the 1.5" height needed to co-witness with the thermal. I'm thinking about a NX8 but value your opinions.

 

[dgheriani]

Pretty much every scope clears with a 1.5" mount.