incorrect behavior in formula

I am using CR XI R2 SP4 on Windows

To have a modular report I have placed common logic in formulas and invoke them as needed in other formulas.

My first question is, can a formula be safely called in another formula?

If yes then it does not yield the expected results. I am using shared variables and these are getting updated when they should not be.

Please let me know the rules for creating formulas that can be called from other formulas.

Regards

Lawrence