Hey @dwaltsch,
I’ve been looking through these myself.
In the example they always use psr compliant classes, I’m not sure if that is the cause, but probably worth the try. You would need name your class CalculateKm and the file should also be CalculateKm.php.
Also, did you remember to run php bin/console cache:clear after adding the class?
According to the following thready you can get some more debug information if set APP_ENV on .env.local to qa. Then on the response you get the error there will be a stacktrace and more info.