Create Calculated Column

Hello everyone.

Please find attached.

I need to calculate the % difference in Transfers from month to month, Per Area.
I've succeeded doing that but I need to somehow test if the Area is = to the PREVIOUS area - then do the some, otherwise, begin afresh (i e dont do the comparison for the first line of the new Area)
Also, if there is only one line for an Area (like DISCOVERY CARD NEW) don't do the comparison at all.

I will eventually pivot this data, so if it is easier to do it in the pivot that will also be fine

will very much appreciate your assistance.

regards
Lorna