Private Function FormulaX(ByVal PAbove As Double, ByVal PBelow As Double, _
                          ByVal TAbove As Double, ByVal TBelow As Double, _
                          ByRef x As Double, ByRef y As Double) As Boolean
 
   Const eps# = 0.000001
   Const add# = 273
 
   'Rückgabe intialisieren   
   x = -1: y = -1
   FormulaX = False
 
   'Parameter-Check
   If Abs(TAbove - TBelow) < eps Then Exit Function
   If PAbove < eps Or PBelow < eps Then Exit Function
 
   'Berechnung   
   Dim zaehler#
   zaehler = (TAbove + add) * Log(PAbove) - (TBelow + add) * Log(PBelow)
 
   y = zaehler / (TAbove - TBelow)
   x = (TAbove + add) * (Log(PAbove) - y)
 
   'Kontrolle
   Dim x2 As Double
   x2 = (TBelow + add) * (Log(PBelow) - y)
 
   If Abs(x - x2) > eps Then Exit Function
 
   FormulaX = True
 
End Function