A calculated custom field is the right choice if an output value should be calculated with a formula. The formula might contain values from source data and/or constant values.
To add a calculated field you need to get to the Mapping step of a task setup and check the section on the bottom-left.
+ sign next to Custom Fields to see options for adding one.
Now select Calculated Field.
To set the calculation formula choose a value from the first dropdown list and click the
+ sign next to it. You will see the value appear in the Preview section below.
Now select an operator and click the according
+ sign to add to the formula.
Repeat Step 2 to add the second value.
Make sure the formula preview reflect the intended calculation and hit Save.
You can edit or delete your Custom Fields in the according section in the bottom-left of the Mapping Screen of your task setup.