ENGINEERING ANSWER
A design Dk of 3.66 is a useful published starting point for standard RO4350B. It is an average from tested material lots and common thicknesses, rather than an exact value for every core, foil and frequency.
What to decide before release
Document the material model used by your simulator. If measured phase disagrees with the model, investigate line length, reference planes, copper profile and launch de-embedding before fitting Dk. Otherwise, an apparent material correction may hide a fixture error.
Checks to put in your design brief
- Keep the original model with the revision
- Use multiple coupon lengths
- Record the final calibrated model
Important distinction
A fitted effective permittivity is not the same as bulk substrate Dk. Microstrip fields also travel through air and any solder mask.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.