PAUT/TOFD...pressure vessel 95mm thick & Cladding thickness 3mm

no doubt you will be inspecting the pressure vessel in accordance with the client-specified code or standard. You should be able to determine the calibration block requirements from that document.
If you are tasked with the such a critical inspection using an advanced instrument such as the Omniscan MX2 you should already be sufficiently familiar with how to set up the instrument. If you are not experienced or qualified to operate the MX2, then the project would be better handled by deferring to an experienced and qualified operator.