Your calculation is valid for air only. For lines in/on PCB material, propagation is slower and the effective wavelength is smaller than the wavelength in air. If the line is completely embedded in a material, effective wavelength = free space wavelength / sqrt(eps_r)
For microstrip, the field is partially in the substrate and partially in air, depending in line width. You can then use a transmission line calculator to get the effective wavelength (or effective eps_r): https://www.microwaves101.com/calculators/1201-microstrip-calculator