How to show last 3 or 6 Months Sales?

1) Create a calculated as below to get the Date values for last 3 or 6 Months.

2) Create another calculated as below to get the Sales values for last 3 or 6 Months.

3) Now add the above 2 calculated fields to the report. But it is not right calculation as explained below.

If OCT, SEP, AUG,JUL,JUN,MAY are as last 6 Months, then OCT, SEP, AUG are Last 3 Months.

When we lace last 3, 6 Months dimension on to the view, tableau is not including the last 3 Months in the last 6 Months,

So we have to take the Running total calculation to get the accurate value.