Try the Function below, will return the number of columns you need to Offset from your Rng to the "Header" you are looking for.

Option Explicit

Function OffesttoHeader(CurrentCol As Long, FindRng As Range, HeaderStr As String) As Long

    Dim HeaderRng As Range

    Set HeaderRng = FindRng.Find(what:=HeaderStr)
    If Not HeaderRng Is Nothing Then
        OffesttoHeader = HeaderRng.Column - CurrentCol + 1
    Else
        OffesttoHeader = -10000 ' raise to a large value >> as an error
    End If

End Function

Test Sub Code (to test the function above):

Sub Test()

Dim ws As Worksheet
Dim Rng As Range
Dim NumberofCols As Long

Set ws = ThisWorkbook.Sheets("Sheet1")  ' modify to your sheet's name
Set Rng = ws.Range("B1:B20")

' pass the following parameters:
' 1. Rng.column - in your case column B = 2
' 2. ws.Rows(1) - the Range to search for the Header, first row in ws worksheet
' 3. "Header" - the Header string you are searching for
NumberofCols = OffesttoHeader(Rng.Column, ws.Rows(1), "Header")

' raise an error message box
If NumberofCols = -10000 Then
    MsgBox "Unable to find Header"
End If

End Sub    

Using the Offset(Row, Column) PROPERTY, you want to offset with the increment of row (i -1), and 3 columns to the right (from column "A" to column "D")

Try the modified code below:

Set example = Range("A1")

For i = 1 To LastRow
    example.Offset(i - 1, 3).Formula = "=SUM(A" & i & ":B" & i & ")"
Next i