See the documentation here: https://docs.mendix.com/refguide/arithmetic-expressions
This appears in the Microflow expressions section of the documentation, indicating that these expressions can be used in microflow actions.
I am not sure if that is the information you are looking for. If not, could you clarify your question?
'Anywhere in a microflow' would be my initial response. You can create a variable in which you include your calculations which you then use, or you can do it in a 'change' action directly.
However anyplace that let's you input an expression is also suited for calculations. Even in the Conditional Visibility property of a page element you could do a calculation.
For readability and easy debugging, I would always do the calculation step by step in a variable(s), in a microflow.