I have coded the following function for my own use. The vast majority of the time, the function will be called from a worksheet cell rather than from another VBA procedure. The function works just fine as written. As a self-taught programmer, I'm wondering if there are some standard programming practices that could improve the code.
1st priority is to minimize round-off error. For example, is there a better way to write the third to last line (change the order of operations or something), to make the calculation more accurate?
2nd priority is improve efficiency.

Thanks in advance for any suggestions.

Typical values for the input arguments:
T as temperature: 100<T<1000
T as pressure: can vary anywhere from 1 to 10E6, typical might be ~100000

Function RIEDEL(T As Double, A As Double, B As Double, C As Double, D As Double, E As Integer, _
Optional slvfrT As Boolean = False, Optional Ti As Double = 300) As Variant
'calculates P0 or T0 using the Riedel equation
'Default, if optional parameters omitted, is to caclulate P0
'If slvfrT is true, then it will find the equilibrium temperature corresponding to a given vapor pressure
'input as T. Ti is an optional initial guess. uses the Newton-Raphson method.
Dim P As Double, T2 As Double, T1 As Double, f As Double, df As Double, itcount As Integer
If slvfrT Then
P = T
T2 = Ti
itcount = 0
itcount = itcount + 1
T1 = T2
f = A + B / T1 + C * Log(T1) + D * T1 ^ E - Log(P)
df = -B / T1 ^ 2 + C / T1 + E * D * T1 ^ (E - 1)
T2 = T1 - f / df
Loop Until Abs(T2 - T1) < 0.000001 Or itcount > 100
If itcount > 100 Then
RIEDEL = CVErr(xlErrNum)
'returns #num error if loop doesn't converge
End If
RIEDEL = Exp(A + B / T + C * Log(T) + D * T ^ E)
End If
End Function