Zhang Heng had made the necessary preparations before he made his move. He knew that if he fell out with Miss F, and even if he successfully dealt with the robot, he would still have to face the attacks of the other police officers around him. Hence, Zhang Heng sent their coordinates and bearings to Miss F and agreed on the location of their meeting.
This wasn't an easy location to choose because once the two drifted apart, Miss F wouldn't be able to support Zhang Heng and Qiu Ming in time. However, if they were close enough and entered the alert area of the two Type-VI Hunting Robots, Miss F and the people she brought would be in grave danger again.
Therefore, they had to accurately gauge the distance between them.