Caesar Cipher Decoding

Caesar Cipher: "em fmzwf qvs ycqkstg" The answer is: "we xerox ink quickly" the most likely decryption key = 8.

Here is the part that I don't understand. I can't seem to get the correct formula working for this, or there is something else I need to do with the results to get the 2nd and 3rd most likely key.

From the cipher text that was produced by the Caesar Cipher above, use the Caesar cipher cryptanalysis technique to find the second and third most likely decryption key i