There is a map which shows you the first bus zones. https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.firstbus.co.uk/uploads/maps/Glasgow-Fare-Zone-Map_web.pdf&ved=2ahUKEwj5v8uoybrwAhVK2qQKHU5kA04QFjAAegQIAxAC&usg=AOvVaw0ZYs7BZZArPH8eapqxp1Pu