Hi Veroli,
You may already know how to create Calculated attribute.
You can create Calculated attribute by selecting below Data Type. Select "Calculated".
After saved Calculated attribute, you will see [Build Calculated Attribute] link in below screen.
To click [Build Calculated Attribute] , you can define your calculation by using available functions.
You can use below functions for your calculation.
For example of my case, I used "Average" as function and custom attribute of num1 and num2 as argument.
Then expression (num1+num2)/2 was created.
Regards,
Shoichi