Within an 1/8 of an inch all the way round so feel good about that.
My chphastria really likes growing on them. Honestly I was tired of trying to stick things to the back wall with super glue and be mostly disappointed. So I thought this was a fun way to build little shelves for encrusting corals to solve that problem that would also provide a fun texture as they grew out. At some point I could choose to cut the shells back off (aquarium safe silicon) and have frags.
I took a risk on black shells given they can leach iron, I will have a fuge that can pull that out though.
I think it will work great but have never seen anyone else do it so maybe it won’t.