When you program the relay, you tell it what CT ratio you used. In the CI problem the ratio is 450:7 (BTW, that's nonsense - ratios are xxx:5 or xxx:1). So if you program the relay to trip at 450A, it will trip when it detects 7A at its input. If you set it for 420A, it will look for 420 x 7 / 450 A, or 6.53A.