You can access and manipulate various internal and user-defined PDFMAILER variables within a calculation.
An input field is provided, once a calculation is added, in which you can define an expression.
You can work with the existing PDFMAILER variables or use your own variables in the field.
Click the Variablesicon 
to select a variable. A dialogue will be displayed, where you can select an existing variable (user-defined or PDFMAILER) or create a new one.
The variables added here can then be manipulated using the following operators:
= (Allocation)
* (Multiplication)
% (Division)
+ (Addition)
- (Subtraction)
For the allocation of texts (in variables of type text), the relevant text must be set in quotation marks (e.g. MyVariable = “My Text”).
In addition to simple calculations, you can also perform complex scripts via the Script API . Click the pen icon 
to open the extended editor.
Script Editor