See if this fits the bill;

Sub SumSelectedCells()
aRandomNumber = 1
CellsToSum = Selection.Address
LastInRange = Selection.Rows.Count
vSum = WorksheetFunction.Sum(Range(CellsToSum))
Range(CellsToSum)(LastInRange).Offset(0, 2).Value = vSum / vRan
End Sub


You can replace the 'aRandomNumber' value with whatever you want!
You can add additional logic to check if the selection IS in columnA with:

vColumnSelected=range(CellsToSum).columns.column
If Column =1 then
SumSelectedCells
endif