I want to know the answer to the following question:
Along with other factors, how long must a target be engaged to have effective ceiling?
All the background I'm provided with is that "By the late 1930s the British definition was "that height at which a directly approaching target at 400 mph (=643.6 km/h) can be engaged for 20 seconds before the gun reaches 70 degrees elevation".". Is that enough to answer the question?
Answer: yes