Modules & VBA :: Argument In Custom Function

I'm having a problem with argument in custom function..My table is like this:

myValues
123.5
32.7
65.8
11.1

What I want to achieve is to multiply each value with the average of all values, so that at the end I get (avg(myValues) = 233.1):

myValues2
7196.963
1905.593
3834.495
646.8525

For this I would like to have a function (this is simplified example of bigger problem, that is why I need function, otherwise simple SQL would suffice as mentioned below also), so that I could do something like this:

'SELECT TestFn([myValues]) as myValues2 From MyTable' or
'SELECT TestFn("[myValues]") as myValues2 From MyTable'

and I should get the table above.

My code is like this

Code:

Public Function TestFn (FieldName) as String
Dim myAvg as Double
myAvg = Davg(FieldName, "myTable")
TestFn = FieldName * myAvg
End Function

I understand the problem is that Davg needs string, but if I change to string then the multiplication does not work. The 'FieldName' parameter does not have a Type specified because that is where the problem is if I give string Avg works ,but calculation does not. If I give Number, avg does not work.

P.S. "as String" at the end of function is not a mistake, I need number as a string

P.P.S I did not inculde NZ either as in my example it can never be Null.