Hi
Correct my understanding, you have restricted a characteristic with a variable and few constants. Suppose if user is not entering the right value, then ignore the input and choose constants. In othercase, it should accept the input value if it is entered correctly?
So it means, you need to validate the user input value and take decision based on that. If it is correct, then try Customer Exit variable (ready for input) and code your logic in i_step3.
Try search in SDN on how to use i_step3 and further details.
Thanks,
Umashankar