Clicking the formula icon will give you 3 options: output as a number; output as a percentage; and output as a currency, or monetary value.
Once this text has appeared in the document, you can navigate through the library to insert fields that will automatically be calculated at the edit document stage.
Note that any values inserted into the formula must be classed as 'numerical'.
Operations supported within the brackets include addition (+), subtraction (-), division (/), and multiplication (*). Calculations will follow BODMAS sequence.