13 Week Moving Average

Hi,

My data looks like this (ignore the values):


I have a Matrix table with Name in Rows, YYYYWW in Columns, WeekWorkHours and 13wmaWorkHours in Values.

For the 13wmaWorkHours (calculated column) i'm using this formula:

13wmaWorkHours =
var selectedweekindex = Table[WeekIndex]
var selectedperson = Table[ID]
return
  SUMX(
    TOPN(
      13,
      FILTER(
        Table
        Table[WeekIndex] <= selectedweekindex &&
        Table[WeekIndex] > selectedweekindex -13 &&
        Table[SAP ID] = selectedperson
      ),
        Table[WeekIndex],DESC
    ),
    Table[WeekWorkHours]
  )/13

I need to do this update: only start counting and filling the column 13wmaWorkHours from the 13th week based on the last 12 weeks. So for the first 12th weeks the column 13wmaWorkHours should be empty and then on the 13th week do the moving average based on WeekWorkHours of the last 12 weeks. I think I also need to find a way to sort the YYYYWW for each employee because the value of 13wmaWorkHours for the first 12 weeks will be empty. 

So the matrix table will look like this:


Only from the 13week of employee Ferk the 13wmaWorkHours value wil appear. 

Any help/tip is welcome. If you need more information/clarification tell me.

Thank you,
Ferk