- Just setup a plain document (e.g. latex)
- ...and include all formulas you need. Encapsulate them into (uniquely numbered) requirements.
- Assign clear and unique names to your calculations and
- refer to them in the UML activity diagrams, state charts, class method names or whatever else you need as artifacts.
You'll probably be faster that way than forcing everything into your diagrams. They're meant to make life easier for you and others, after all ;)