Andre,

I agree with the things you wrote except the following: "But a return calculation should not depend on the units used for the values."

In my opinion, a return calculation is heavily dependent on the units used for the values. For a result to be measurable and comparable, the data or unit needs to be similar.

Unless we only focus on measurability and not comparability, then the equation Michael wrote could be use (although I won't recommend others to use it).

tyc