Calculate column difference using a already calculated column

Hi all, I'm having trouble calculating the difference between two columns in my visual. In table 1, one column is called Average Unit cost (which works out the average using Total Cost and Total Activity, both columns in same table) and in Table 2 I have another column called NationalAverage. It seems simple but I cannot seem to create a measure or column that works out the difference between the two columns. Any help would be great!

Thanks

Sunny