At the start of the movie, they are 30 years underway. From that position, the signal takes 19 years to reach Earth. That means, with Newtonian physics, they are 19 light years (ly) from Earth. Average speed = 19 ly / 30 years travel, so about 63% of light speed.
For the return signal, arrival is 55 years later (movie is not clear if this is 55 years after arrival at Earth, though). If the signal returns 55 years after being sent, return travel time is 36 years, or 36 ly from Earth. If 55 years from Earth, distance is 55 ly from Earth, but arrives 74 years after being sent. In the first case, it travelled 17 ly in 55 years (31% light), in the second, 36 ly in 74 years (49% light). Both of these answers are inconsistent with the initial condition of travelling 19 ly in 30 years.
At these speeds, relativity kicks in, with shipboard clocks running slow relative to Earth, or conversely, Earth appears closer. At 50% light, for example, the dilation is at 14%. And at this point, I end up confused on how to apply it!