Hi MJ,
The best way here is to use a monthly time bucket profile (on this same Planning Book or in another one) to see this data.
Anyway if your business requirement is to use a Macro, you may try these funcsions:
PARTIALSUM( X ; Y1 ; ... ; Yn ) allows you to sum a variable number of values in a time series where X specifies the number of values and Y1 through Yn specify the time series.
SUM_CALC( key figure ) calculates the sum of all detail levels of the specified key figure.
Best regards,
Leonardo