The IOR is an exact value that you should look up for each material. Although, the default glass IOR is probably accurate.
List of IOR values:
The index of refraction refers to the angle that light propagates through a surface. It is a different thing than the amount of refraction you see.
The thickness of the tube changes it’s amount of refraction:
Also, it’s very important to orient your front faces outwards before rendering (even in V-Ray).
Does that solve your problem?
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