Hi Florian,
predefining complex business calculations according to company's definition in the model however does not hinder the user to create his own calculations. But in this case the user is responsible for his calculation which may differ from the companies definition. As long as all source columns/measures are visible to the user you do not limit the end user in any way.
But coming back to the original question, we can say that it is not possible to define a universal average/ratio/... calculation in an analytical view - correct?
-gerhard