Morse characters have different lengths. A series of Es is much shorter than a series of Os. Therefore, the WPM is perhaps a better choice, because a word contains different characters.
Based on the two standard 5 character words "Paris" and "codex" and their dot-dash-ratio, the WPM/CPM factor has been calculated as 5. So
- 6 WPM = 30 CPM
- 10 WPM = 50 CPM
- 12 WPM = 60 CPM
- 20 WPM = 100 CPM
Wikipedia has this explanation, but does not further specify who first came up with these two words:
There are two common typical words: "PARIS" and "CODEX". PARIS mimics a word rate that is typical of natural language words and reflects the benefits of Morse code's shorter code durations for common characters such as "e" and "t". CODEX offers a word rate that is typical of 5-letter code groups (sequences of random letters).
The [Federal Communications] Commission specifies Morse code test elements at 16 code groups per minute, 20 words per minute, 20 code groups per minute, and 25 words per minute. The word per minute rate would be close to the PARIS standard, and the code groups per minute would be close to the CODEX standard.