Calculating Standard Dev from an aggregate or measure

Hi Guys

I have spent over past 2 days trying to figure out a solution to this problem, i have been asked to create a SPC by a client, in which the main table is in this a raw format with multiple rows i.e customer name, day, number of sales etc

C TABLE

Customer name, day, number of sales

A                       1               10

B                      1               10

C                      1                20

A                      2                10

B                      2                 40

So i have managed to create an aggregate table using mixture of different measures which looks like this with day as filter

Customer name        Number of sales            Mean             My ideal answer

A                              20                                  30                   13.33

B                               50                                  30                   13.33

C                               20                                 30                    13.33

                                  90                                30

Mean is calculated using measure

M Measure

Total number of sales = SUM(C[Number of Sales])

DISTINCT COUNT of CUS = DISTINCTCOUNT(C[Customer name])

Mean = CALCULATE (DIVIDE(M[Total Number of Sales], M[DISTINCT COUNT of CUS], ALLEXCEPT(C, C[Day]))

I have tried to apply the similar logic to work out Standard Derivation using STDV.P against C[Number of sales], but realised that the STDV.P is calculating the derviation of the C table and not the aggregate table. 

So next i have tried using the same approach with SQRT((Total Sales - Mean)^2) - but realised that the result showing as

A       SQRT( (20-30)^2)

B        SQRT( (50-30)^2)

C        SQRT( (20-30)^2)

          SQRT( (90-30)^2) <<<< which is wrong it should be average of all above

My ideal answer for all rows for the aggregate table should be 

AVERAGE( SQRT( (20-30)^2) + SQRT( (50-30)^2) +  SQRT( (20-30)^2)) which is STDV.P on my aggregate NOT my table

I am getting to stage of giving up and would be grateful if any guru or master can help!!!!!!!!!!!!!!!!!!   :smileysad:

Happy to share the file