Problems with code for keeping running total

Are your sheets in order that you could add value from previous sheet?

for example, if you have number of sheets:

 n = Worksheets.Count

and new sheet is last (n-th), then you could add

Worksheets(n).Range("B26").value = Worksheets(n).Range("B26").value + Worksheets(n-1).Range("B26").value

Does that help?

Thanks Zbor,
All years are in order with year end summary's in between. I am only using this VBA code starting February of 2013 going forward.

If i was going to try this, would i just drop your code in as i have shown below.
I used 12 for the value of N for the 12 months of the year. How would this work for the 1st month, if the year end summary was present? And is 12 the correct number to use or should i enter in 24 for the next 2 years i have created sheets for?

Private Sub Worksheet_Change(ByVal Target As Excel.Range)
      With Target
         If .Address(False, False) = "B25" Then
            If IsNumeric(.Value) Then
               Application.EnableEvents = False
               Range("B26").Value = Range("B26").Value + .Value
               Application.EnableEvents = True
               Worksheets(12).Range("B26").value = Worksheets(12).Range("B26").value + Worksheets(n-1).Range("B26").value
            End If
         End If
      End With
    End Sub

You can also set worksheets at beggining so it won't influence if you run macro in wrong sheet.
For example

Dim wslast as Worksheet, wsprevious as Worksheet
Dim n as integer

n = Worksheets.Count
Set wslast = Sheets(n-8)
Set wsprevious = Sheets(n-9)

 If IsNumeric(.Value) Then
               Application.EnableEvents = False
               wslast.Range("B26").Value = wslast.Range("B26").Value + .Value
wslast.Range("B26").value = wslast.Range("B26").value + wsprevious.Range("B26").value
End if

Hard to say without sample workbook: depends how you add sheets. Worksheet(1) is first worksheet. Can be some Summary also, not only your data.
Worksheet(2) is second sheet.. And so on...
Worksheet(12) should be variable, not static. So if you have in total n worksheets (for example 20), and you dealing with 12-th (because you have some additional sheets) then your formula would look something like:

 
Dim n as integer
n = Worksheets.Count
Worksheets(n-8).Range("B26").value = Worksheets(n-8).Range("B26").value + Worksheets(n-9).Range("B26").value

Because 12 is 20-8, and previous is 20-9.
Of course, if yoiu add sheets in reverse order you must reverse your logic.

Thanks again Zbor, but its all a little beyond my grasp. Ill either leave it as is or just go back to the manual method.