To do operations between variables you can use calc($var_name [operation] number) in the variable description.
You can do addition, subtraction, multiplication and division.
e.g: calc($radius * 1.4)
If the name of your variable has spaces, replace them with underscore _,
e.g: calc($var_with_space + 1.4)
Apply operations
To apply the operations you must go to the plugin UI and in the “Variable calculations” section you have two options:
Start monitoring: This option will be listening all the time for changes in the variable descriptions and will automatically apply the operations until you press Stop monitoring.
Update once: It will do the calculations only once, if you make a change in the operations you will have to press this button again to recalculate.
Important note: The monitoring may consume resources of your computer, if your computer is not very fast it is best to use update once.