Power Pivot Measure for Month over Month Variance

Hi, I have recently converted a revenue table from a regular Pivot Table to Power Pivot.

It's a simple customer down the left and monthly periods across the top, which shows revenue by customer / product etc for the last 12 months.

I am now struggling to create a measure to show the variance between the current month and the previous month (month over month variance), which would show in the right hand most column (similar to what is available as a calculated field in a regular pivot table).

Any guidance please?

Thanks!