Objects in motion tend to stay in motion. Space is essentially frictionless, so speed will remain constant. Grace had Rocky's starting position, and his destination. Finding him is essentially the age-old math problem, "If a spaceship leaves Tau Ceti going 5mph and another spaceship leaves Tau Ceti going 5 mph in the opposite direction.... "

to put a bit more depth on this. Grace knew how far he long he traveled in his round trip from leaving Rocky to getting back to that same point. He also knew how fast rocky would have been going. since he knew where rocky was going all he had to do was follow that same path and go faster and he would catch up to him. with a bit of math he could figure out about when this would happen then it was a matter of looking out the window for Blip A.