Hi everybody !!!
You should pay attention to the 'real' stackup of your PCB, and especially the resin content.
Most of the time, the specification are given for 'pure' material.
When the PrePreg is fabricated, there is some resin is added to glue the fibrous laminate. The resin has a different dielectric constant, and the actual dielectric constant vary, depending on the resin content.
You have to call your PCB manufacturer to have the actual values...
In fact, there is a dielectric constant for each laminate thickness for a given material.
Here is a datasheet of High Tg PCB material from isola.
You could see that the impact of the resin content of the dielectric is more important than the frequency....
I 'm not designing critical High speed PCB, but I have heard that for small series, the manufacturer buy a stock, make measure, and then adapt the design to the characteristics of the stocked material...