Have a look at this:
Recursive Calculations in PowerPivot using DAX | Gerhard Brueckl's BI Blog
Your situation is similar but you want to look back a year so you would use SAMEPERIODLASTYEAR in your formula.
Or possibly a pattern like this might work (if you are happy to forecast no more than 12 months into the future):
Code:Sales Forecast = SUMX ( VALUES ( 'Calendar'[Month] ), IF ( ISBLANK ( [Sales Actual] ), CALCULATE ( [Sales Actual], SAMEPERIODLASTYEAR ( 'Calendar'[Date] ) ) * 1.15 ) )