Assistance with DAX Calculation for Previous Month Values

Hi Community Members,

I’m using the following DAX to calculate the value for the previous month. If the previous month's value is unavailable, it should default to the second previous month's value, and this logic should continue for up to 13 months to retrieve the correct values.

The DAX below works fine for individual calculations, but I’m encountering an issue where the total is not adding up correctly.

Value Last 13 Months Previous Month =
SUMX(
VALUES('Calendar'[Date]),
VAR Month1 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
PREVIOUSMONTH ( 'Calendar'[Date] )
)
VAR Month2 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -2, MONTH )
)
VAR Month3 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -3, MONTH )
)
VAR Month4 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -4, MONTH )
)
VAR Month5 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -5, MONTH )
)
VAR Month6 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -6, MONTH )
)
VAR Month7 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -7, MONTH )
)
VAR Month8 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -8, MONTH )
)
VAR Month9 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -9, MONTH )
)
VAR Month10 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -10, MONTH )
)
VAR Month11 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -11, MONTH )
)
VAR Month12 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -12, MONTH )
)
VAR Month13 =
CALCULATE (
SUM ( vw_Report_1241[Total_AM] ),
DATEADD ( 'Calendar'[Date], -13, MONTH )
)
VAR Result =
IF (
NOT ISBLANK ( Month1 ),
Month1,
IF (
NOT ISBLANK ( Month2 ),
Month2,
IF (
NOT ISBLANK ( Month3 ),
Month3,
IF (
NOT ISBLANK ( Month4 ),
Month4,
IF (
NOT ISBLANK ( Month5 ),
Month5,
IF (
NOT ISBLANK ( Month6 ),
Month6,
IF (
NOT ISBLANK ( Month7 ),
Month7,
IF (
NOT ISBLANK ( Month8 ),
Month8,
IF (
NOT ISBLANK ( Month9 ),
Month9,
IF (
NOT ISBLANK ( Month10 ),
Month10,
IF (
NOT ISBLANK ( Month11 ),
Month11,
IF (
NOT ISBLANK ( Month12 ),
Month12,
Month13
)
)
)
)
)
)
)
)
)
)
)
)
RETURN
Result
)