Rolling Average for Future Months

Firstly, here is a method of calculating an X Month average.  I've assumed you have a selection table and made a measure called "YourSelection" that handles how many months you are considering for this average.  I'll explain the selection table next.

YourMeasure X Mth Avg = CALCULATE( 
	if(
		countrows(values('Dim - Date Table'[YearMonth])) = 1, 
		[YourMeasure], 
		AVERAGEX(
			values('Dim - Date Table'[YearMonth]), 
			[YourMeasure]
		)
	),
	DATESINPERIOD(
		'Dim - Date Table'[Date], 
		LASTDATE('Dim - Date Table'[Date]), 
		[YourSelection], 
		MONTH
	)
)

Now the selection table could be anything, but the purpose is create a slicer that links to this table and allows the user to pick.  This could be a list of numbers from 1 to 12, or a list of words like "Last Quarter", "Last Month" with corresponding numbers.

Then this selection is transated using:

YourSelection = MIN('Selection Table'[NoofMonths])

So now you have an X Monthly average.  Do something like

NextMonthPredict = ([YourMeasure X Mth Avg] / [Targeted]) * [NextMonthTarget]

Here i assume you have a measure that knows what you targeted each/current month as and that you can calculate next months target.