Calculating rolling average based on a expression and connection with a second data table

Hi all,

My data is as follows:

I have 2 Date Tables, one is called 'D Kalender'  and the other is called 'D Kalender Previous'. The 'D Kalender Previous' is table which is a copy of 'D Kalender' table. Both tables are connected via *-1 relationship from 'D Kalender' to 'D Kalender Previous', based on [TID_ID] that's inactive.

I've done this because I have build 1 report, with many visuals and some visuals I want to show only based on the selected period (monthyear) and others I want to show from the selected period-13 months to the selected period. For the first kind of visuals I have 'D Kalender[MonthYear]' are tables or cards without x-axis and shows only data based on the selected month or for example YTD calculated by a formula based on the selected date. For the second kind of visuals that are mostly line charts, where I have 'D Kalender Previous[MonthYear] on the x-axis. So that I see for example not only 1 month, but the values of the last 13 months. Based on this video https://www.youtube.com/watch?v=d8Rm7dwM6gc

Now I have a measure to calculate the actuals, called [Actuals], that's a formula that takes the sum of a lot of values. Without filtering on dates. This measure I used for the first kind of visuals. For the second kind of visuals I have use a newly created measure called [Actuals siste 13 månader avhenging av valgt dato] to show the actuals each month. And this works fine!

The measure is as follows:

Actuals siste 13 månader avhenging av valgt dato = 
VAR ReferenceDate = max('D Kalender'[Dato]) // selected date
VAR PreviousDates =
DATESINPERIOD(
'D Kalender Previous'[Dato],
ReferenceDate, // fra selected date
-13, // til 13 måneder tilbake
MONTH
)
VAR Result =
CALCULATE(
[Actuals],
REMOVEFILTERS('D Kalender'),
KEEPFILTERS(PreviousDates),
USERELATIONSHIP('D Kalender'[TID_ID], 'D Kalender Previous'[TID_ID]) 
)
Return
Result

Now I want that to create a new measure which calculates the rolling average for the last 12 months for those values, so that I can add that as a line to the same visual. But when I try to calculate that, I don't get the result I want to achieve.

I've tried now to use the following formule

Actuals 12 mnd rullerende snitt avhanging av actuals siste 2 års = 
VAR NumOfMonths = 12
VAR SelectedDate = MAX('D Kalender Previous'[Dato])
VAR Period = DATESINPERIOD(
'D Kalender Previous'[Dato], SelectedDate, -NumOfMonths, MONTH)
VAR StartDate = FIRSTDATE(Period)
VAR Result = 
AVERAGEX(Period, 
CALCULATE(
[Actuals],
REMOVEFILTERS('D Kalender'),
KEEPFILTERS(Period),
USERELATIONSHIP('D Kalender'[TID_ID], 'D Kalender Previous'[TID_ID]))
)
RETURN
Result

With as result for each MonthYear the values of the actuals in stead of the rolling average for the last 12 months.
So I get now this:

But I had expected, and I want to achieve

How can I achieve this? I hope that someone can help me.