Interleaved 2 of 5 uses a check digit as the last character, at least that is what is recommended. There is supposed to be an even number of characters in the code. The check digit reduces the chance of a bad read by a factor of 10. It is somewhat application specific. Your system should expect a fixed number of digits.
Less digits means a bad read.
Make the calculation as simple as you can. Maybe add all of the digits and append the least significant digit of the sum to the desired bar coded number. I believe it is up to you. The specific answer is that you have to know how the check digit was originally calculated. If there is no one to ask, get 20 or more samples and figure it out.