The gas pump islands are perpendicular to the main road so as the cabbie is pumping gas, he is standing between the car and the pump. He was shot in the chest. The pumps are head high and the the car blocks the other direction. That means the shot came from the direction of the parking lot. Because the parking lot sits about 10 feet above the gas station grade, he couldn't stray too far from the road before he loses his line of site to the gas pumps.
I heard that the police were searching the rooftop of the Giant Food store (large dark gray roof top at the bottom center) and if he wasn't near the end of the parking lot that would be the only other place he could get that shot off from. I have a hard time believing that he could get on the roof and off without being noticed and then get up the street to Leisure World and set up his next shot 25 minutes later. Anythings possible I guess...