I'm not sure why you've mentioned column B, as you never use it again. I
suspect that your explanation might be incorrect. However, I've taken it
exactly as you had it. I.e. use the selected range to position a formula in
C at the bottom of the selection and the sum is from column A. I'm also
assuming that the number you want to divide by is in F1.

Option Explicit

Private Sub CommandButton1_Click()
Dim TopRow As Integer
TopRow = Selection.Row
Dim Height As Integer
Height = Selection.Rows.Count
Dim BottomRow As Integer
BottomRow = TopRow + Height - 1
Range("C" & BottomRow).Formula = "=sum(A" & TopRow & ":A" & BottomRow & ")
/ $F$1"
End Sub

The only unusual thing I've done is to ignore part of the current selection.
I get the top row's number, plus the number of rows in the selection,
allowing me to determine the bottom row. I then force the summed range to
be in column A and the put the formula in column C. This means that you
don't actually have to select the correct column before hitting the button -
just make sure that the right rows are in the selection.