Do I find the Coincidence Index for a single letter and the general one in the Vigenère cipher correctly?

Question

Encryption

Hello.
I'm trying to determine the length in the text encrypted by Vigenère.
Do I find the Match Index for a single letter correctly, the formula is: n*(n-1)/L*(L-1) for each letter.
Just as a result, if a letter occurs only once, then its IP will be = 0, is this normal?

Also, tell me, did I understand correctly that the total IS of the entire line will be the arithmetic mean of all the IS of the letters?

Thank you in advance.