I have a problem with the 'light clock' operation... postulate 2 says that light is propagated in free space with a defined speed that is independent of the emitting source... velocity is a vector quantity, ie it is directional. so in order for the velocity to remain independent of the emitting source, the direction of propagation must not be affected by the motion of the emitting source. therefore according to postulate 2 the motion of the emitter will have no effect on the direction of the light it emits and therefore the light will have no vector component induced by the motion of the emitter and will not travel parallel to the emitting source. the light will travel in a straight line from the point in space where it is formed, and propagate in the direction the emitter is pointed at that instant. only a particle with mass, can and will, have momentum transferred to it, thereby causing a vector angle path whereby the particle will travel parallel to the emitter. light must travel in a straight line from the point in space where it is created. so, any frame of reference that is moving with respect to that point in space where the photon is created will see an angled path for perpendicular movement and Doppler effect for parallel movement(same or opposite direction). creating a photon in free space with a moving emitter is like slapping the surface of the water on a lake while traveling along in a canoe, where the created waves propagation is independent of the emitters/slappers motion. creating an EM wave with an omnidirectional antenna while traveling in a spaceship at high velocity would give a similar visualization as the canoe experiment, but since light is a unidirectional EM wave it would travel from the point of creation in space and propagate in the direction the emitter was pointing(perpendicular to ship travel) and not in a vector angle direction from the emitter. so, the light clock could not work as described by Einstein et al if postulate 2 is true..... Either the postulate or its application in the light clock thought experiment is incorrect...
Einstein was wrong, one way or the other...
however, to answer your question, time dilation is the Pythagorean ratio of velocity of an object to c within a given medium. you cannot compare the velocity of light between mediums and infer a dilation.