The British adopted "effective ceiling", meaning the altitude at which a gun could deliver a series of shells against a moving target; this could be constrained by maximum fuse running time as well as the gun's capability. 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". However, effective ceiling for heavy AA guns was affected by non-ballistic factors:
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): Along with other factors, how long must a target be engaged to have effective ceiling?
for 20 seconds