Exactely. The Er you'll better obtain it from your pcb manufacturer. Roughly you can consider Er for FR4 at 4,6-4,8, and loss tangent around 0.02 . Often, pcb manufacturers have application notes for their substrates in which physical parameters are given for various trace impedances. Ask them, maybe this will spare your time. Another issue would be the frequency of the signals. In high speed design, is not important the maximum clock frequency you use, it is important the highest frequency component which will appear in your system, and this is F=1/2t , where t is the fastest rise or fall time (whichever is less) in your system.