Object-level business rule to calculate field

Hello Community!

I'm trying to fill a field with the result of a simple calculation (e.g. field A - field B).

However, when I create the business rule and select "set field" as action, I can only provide a constant value and not a formula...am I missing something?

Here is a screenshot of the 

I know that I can use a business process to achieve this, but this would be much more simpler and elegant I think.

BR,

Robert