Consegui dessa forma.
Media de vendas ultimos meses =
CALCULATE (
AVERAGEX (
VALUES ( dCalendario[Ano e Mês] ),
[Total de Vendas]
),
DATESINPERIOD (
( dCalendario[date]),
EOMONTH ( MAX ( dCalendario[date] ), -1 ),
SWITCH(VALUES(dCalendario[Mês Num]),
12,-11,
11,-10,
10,-9,
9,-8,
8,-7,
7,-6,
6,-5,
5,-4,
4,-3,
3,-2,
2,-1),
MONTH
)
)