Here is the base, as of January 31st.
As you can see:
(7180) Federer Minimum
 Nadal Maximum
 Djokovic Maximum
 Del Potro Maximum (including 2 optionals)
 Murray Maximum
Djokovic was third at that point with pontential points.
I don't understand anything about your calculations. I never had this result : Djokovic was always ahead of Murray in mine, together with Davydenko and Del Potro.
Midnight Ninja's suit me very well.