Option Explicit
Sub Rechnen()
Dim Bereich As Range
Dim Start As Range, sucheG1 As Range, sucheG2 As Range, Suchbegriff As String
Dim ZeileG1 As Long, ZeileG2 As Long, AnzahlZeilen As Long
Dim ErsteAddresse As String
Suchbegriff = "G"
With ActiveSheet.Range("A1:A500000").Cells
Set Start = .Find(Suchbegriff, After:=Range("A1"), LookIn:=xlValues)
Set sucheG1 = Start
ErsteAddresse = Start.Address
Debug.Print ErsteAddresse
Do
Set sucheG2 = .FindNext(sucheG1)
ZeileG2 = sucheG2.Row
ZeileG1 = sucheG1.Row + 1
AnzahlZeilen = ZeileG2 - ZeileG1
sucheG1.Offset(0, 3).FormulaR1C1 = "=SUMIF(R[" & 1 & "]C[-3]:R[" & AnzahlZeilen & "]C[-3],""P"",R[" & 1 & "]C:R[" & AnzahlZeilen & "]C)"
Set sucheG1 = sucheG2
Loop While sucheG1.Address <> ErsteAddresse
End With
End Sub