If certain calculations are set up via rules, then you'll need to make sure that the rules are triggered every time they are needed. For example:
field3 = field2 * field1
If you have a rule on field1 that performs the multiplication, then you also need the same rule on field2. The reason is that you never know whether the user will enter the value for field1 or field2 first.
You can extend this to other scenarios. For example, if a field is calculated as a result of 3 other fields, all three of the fields will need the calculation rules set up.
Hope this helps.