A book like Microwave Engineering by Pozar would help you learn more about the topic. There are equations for the effective dk in Pozar's book. A quick and dirty calculation is to average air (medium on top of the transmission line) with the dk of the material on the bottom of the transmission line. The dielectric in microstrip is important because it allows you to manufacture the transmission line. It's hard to build a transmission line with air as the dielectric medium. Because the material properties are different then air, you need to take them into account. The dk along with other parameters will determine the impedance of the transmission line.