Need help to create the DAX

[Avivek]

I have a projection measure, previously it used to calculate the actual proj month usage + last 2 month usage, i want to make few changes to it. Instead of the last 2 month i want to use last n month usage. The last n month usage  should use the following logic-

1. When Oct is selected, show only Oct projection
2. Sep and Oct, show only Oct projection as Tot.Proj
3. Aug, Sep and Oct, show Sep+Oct as tot proj
4. July, Aug, Sep and Oct, show July+Sep+Oct as tot proj  (current prod)

The current measure I use is -

Tot. Act. proj. Mon. Usage = [Act. proj. Mon. Usage]+[Act. Last2 Mon Comp.Usage]

Act. Last2 Mon Comp.Usage = CALCULATE(count('DO Vs Tag'[Procedure Credit Owner]), FILTER('DO Vs Tag','DO Vs Tag'[Last 2 Mon Procedure]=1))

The measure I need to modify is Tot. Act. proj. Mon. Usage.