Controlled Impedance Microstrip Frequency
Dielectric constant for most PCB materials fall with increasing frequency. This is a logarithmic curve, so in practice it's fairly flat from 1-2GHz and up. That is the reason I would calculate characteristic impedance at 1-2GHz and be happy with that even at much higher frequencies.

Notice that you may easily have +/-10% production tolerance on characteristic impedance in inner layers (for narrow traces) and maybe +/-20% on outer layers. So don't overdo this.

Sorry for the late answer - hope it's still useful.