How to use the Before function to subtract Month 1's value from Month 12's value of previous year.

[Marisa]

I am working with a financial year calendar which starts at 01/10/2019 and ends at 30/09/2020. At the end of each month an asset gets a Reading Value. I want to display the difference in reading values each month.

As seen in the screenshot - the 1-10 is indicating the Months. 1= Oct, 2 Sept etc

The value under the month's number is the calculation being performed: [Monitoring Point Reading] - before([Monitoring Point Reading]) For e.g. Month 2 (11/30/2019) = 200-99 (as seen in screenshot below).

I want to see October's (Month 1) value as well, that will be 9/30/2019 - 10/31/2019 (99-50), but Sept is part of the previous Financial Year and is Month 12. Therefore, when using the 'Before' function the first value is NULL because it doesn't pick up the previous years end as its predecessor.

Thanks is advance

Marisa