Erlang C algorithm

Hi everybody,

I already wrote the algorithm (a few years ago) in VBA for Excel to calculate the number of agents needed with the following parameters:
Nb of calls / hour
Productivity (number of calls that can be answered in an hour)
Quality of service (percentage of calls that have to be answered)
This is the basic way.
Here is the algorithm for those who are interested :

Function NbLignesNecessaires(A As Double, T As Double) As Integer

Dim i As Integer
Dim c As Integer
Dim Num As Double, Den As Double, D1 As Double, D2 As Double
Dim Temp As Double
Dim test As Integer

Temp = 2#
c = 2
Taux = T

While Temp > Taux Or Temp < -Taux

Num = (A ^ c / Factor(c)) * (c / (c – A))
Den = 0

For i = 0 To c – 1
Den = Den + A ^ i / Factor(i)
Next i

D1 = Num / (Den + Num)
D2 = c * T / (c – A * (1 – T))
Temp = D1 – D2
c = c + 1

Wend

NbLignesNecessaires = c – 1

End Function

I would like now to modify it and add the Grade of Service ; that means the percentage of calls that have to be answered within a target, for example 85% within 25s.

Could you help me ?

Thanks a lot in advance,