Creating a calculated variable is done in exactly the same way as a variable of any other type.
In DataDynamic if you have a file open:
1. go to the 'editing' tab and click on 'add' in the 'variables' section in the top menu. If you have no variables selected then the type should automatically be set to 'Calculated variable'
2. Select the base data type for your variable to be one of 'Numeric', 'Text' or 'DateTime'
3. Define a name for the variable, the check mark at the end turns green if you have specified a valid variable name.
4. Optionally define a description for the variable
5. Specify the formula to use for this calculated variable, just under the formula text box you will get feedback on the formula you have entered. To try it out you can specify the name of a different variable already present in your dataset and of the same base data type.
6. Click on 'Create variable' to add the variable to the dataset, this is only possible if yu have specified a valid name and a correct formula.
Your variable is now available for use in DataDynamic just like any other variable.