weighted average for a month

Hi guys, I have a problem with my measure, 

I would like to do a progressive calculation. That is to say I have three columns, one with the number of session, one with the average time per session and another with the cumulative number of session each month.

Starting from a date, I would like to calculate a weighted average on a month period. For each day from the date I started to the last month, i want to divide the number of sessions of each day by the cumulative number of session each month equal to the DAY I STARTED. The total multiplied by the average time per session. 

DIVIDE(
	CALCULATE (
	    SUM ( 'Google analytics'[Number of sessions] )
	        * SUM ( 'Google analytics'[Average time per session] ),
	    DATESINPERIOD (
	        'Google analytics'[Date].[Date],
	        FIRSTDATE ( 'Google analytics'[Date] ),
	        -1,
	        MONTH
	    )
	),
	 SUM ( 'Google analytics'[Cumulative number of session each month] )
)

To give you an exemple I am giving you this excel:

I want to calculate the session time weighed average for the DAY 6, so i have to sum all the session time weighed average of all other days. To calcul them, i am using the last cumulative number of sessions. 

I hope you can help me, tell me if i'm not clear enough.