You can determine the average by looping through the Array and computing the
arithmetical mean....

For x = 1 to 50
  xSum = xSum + myArray(x)
Next x
xMean  = xSum / 50

(above assumes Option Base = 1)

The question posed, to calculate a moving average, depends on the ability to
track the last value in the array. The array has 100 values, any or all of
which could be set to zero.  You could scan the array looking for the first
empty value and then use that to compute the mean of the preceding 50
values.

Sub aTest()

Dim x As Integer, LastEntry As Integer
Dim xSum As Double
Dim xSpan As Integer

xSpan = 50  ' sets the number of values to compute the mean over

' determine the last non-empty value in the array
For x = 1 To 100
  If IsEmpty(myArray(x)) Then
     LastEntry = x - 1
     Exit For
  End If
Next x

' compute the mean for the last 50 ( xSpan) of values in array
If LastEntry >= xSpan Then
   For x = LastEntry To LastEntry - xSpan + 1 Step -1
         xSum = xSum + myArray(x)
    Next x
     MsgBox "Mean: " & xSum / xSpan
Else
     MsgBox "Insufficient entries to compute"
End If

End Sub