How to fix this true value in my Public Function?

[kingman29]

Hello
First I want to thank @JackDanIce for gave me an excellent solution
Second I marge his function (his solution) into my function.
So
When I tried to calculate The "Percentage_CP" must be this value =

0.001256831​

Witch mean the Result in Cell "E6" will = 588 404.74630183
But it show me this =

0.001255254​

So this this Result = 587 666.149010485 is Fault not true
so How to fix it ? please
thank you veryMatch

*************************************

boutton.xlsm
	A	B	C	D	E
1	Cost1	-411 269 098.48
2	Pourcentage	0.14%
3
4	Devesement Pourcentage	0.13%
5
6				Deversement	587 666.15
Feuil1

Cell Formulas
Range	Formula
E6	E6	=deversement(B1,B2,B4)

****************************

The function in the Module is

Public Function deversement(Charge_Centre_Principal, Centre1 As String, Percentage_CP As Double) As Double
Dim y As Double
    For y = 2 To 40
        deversement = (-Charge_Centre_Principal * Centre1) * (1 + TauxCP + Application.Power(TauxCP, y))
    Next y
End Function

 

[GWteB]

There's a missing variable, so the used one will initialize to 0 (zero). Try this:

Public Function deversement(Charge_Centre_Principal, Centre1 As String, TauxCP As Double) As Double

Dim y As Double
    For y = 2 To 40
        deversement = (-Charge_Centre_Principal * Centre1) * (1 + TauxCP + Application.Power(TauxCP, y))
    Next y
End Function

 

[kingman29]

There's a missing variable, so the used one will initialize to 0 (zero). Try this:

Public Function deversement(Charge_Centre_Principal, Centre1 As String, TauxCP As Double) As Double

Dim y As Double
    For y = 2 To 40
        deversement = (-Charge_Centre_Principal * Centre1) * (1 + TauxCP + Application.Power(TauxCP, y))
    Next y
End Function

The Result is =

588 403.82​

It's wrong result

 

[GWteB]

You are not declaring the Charg_Centre_Principal variable explicit as a Double type, so it implicitly would be a Variant type holding any type.
Furthermore, you're declaring the Centre1 as a String type variable, despite it's used for calculations. Best would be to declare it as a Double type.
Don't know if this resolves your problem. I do have some VBA knowledge but (sorry to say) I'm not a math geek.
Hopefully some other forum member is able to help you with that part.

Public Function deversement(ByVal Charge_Centre_Principal as Double, ByVal Centre1 As Double, TauxCP As Double) As Double

Dim y As Double
    For y = 2 To 40
        deversement = (-Charge_Centre_Principal * Centre1) * (1 + TauxCP + Application.Power(TauxCP, y))
    Next y
End Function