Does anyone know the correct size for this cap for use with a Ribelle ISP?

Dedacciai's catalog lists 3 versions: 35.0 - 35.3 - 35.8 mm

I plan on replacing it with a Ritchey cap as they provide an 8x8.5 clamp for use with Sella Italia carbon rails. Thing is they only make a 34.9mm size however they aren't the only vendor providing these.

The standard (non ISP) version of the frame specifies a seat tube for 31,6 mm ID seat post

Thanks...Rick