The dashboard below shows Inventory Items with their 'MaximumLevel' buffer and the current 'QtyOnHand'. Lets add a field to show the difference between these 2 values.
Navigate to the 'Data Source' tab and click on the 'Add Calculated Fields' button.
This will open the 'Expression Editor'. This expresses the difference between the 'MaximumLevel' field and the 'QtyOnHand' field.
The newly created field will appear in the data tree on the left of the dashboard. Right-click on it to 'Rename' it. I will call this field 'Buffer Difference'.
Drag and Drop the newly calculated 'Buffer Difference' field as a Data Item under 'Columns'. This will add the 'Buffer Difference' column to the grid.