Calculated field in pivot table

Hi,

I have a pivot table where the invoice values are shown by month and year. I managed to write an expression to calculate the delta percentage: ([Current Year]-[Previous Year])/[Previous Year]. This is being displayed in a separate pivot table.

Now to save some space on the sheet and to make it visually more appealing I would like to incorporate both the invoice values and the delta percentages in one pivot table. As you can see in attached print screen simply pasting the delta percentage expression did not do the trick.

Does someone have a solution for this?

P.S. I already found some information in another thread (Add a calculated column to pivot table with set analysis) where the solution of a dummy dimension is mentioned. Unfortunately I do not understand how this works.