I found this map of the Galaxy with the new planets on it (apparently, it's from a special Entertainment Weekly edition that came out recently)...
Based on this, Scarif is very close to Tatooine. Leia's ship may have already come out of hyperspace near Tatooine by the time Vader got back aboard his Star Destroyer. In that case, they wouldn't have needed to try to track its trajectory or anything, just jump right there and stop the ship before it gets to the planet's surface.