ABCDEFG estimations are based on classes. Teams are divided to 7 classes, depending on number of points / number of games ratio:

Class A: > 2,00
Class B: > 1,75 & < 2,00
Class C: > 1,50 & < 1,75
Class D: > 1,25 & < 1,50
Class E: > 1,00 & < 1,25
Class F: > 0,75 & < 1,00
Class G: < 0,75

Lets check few teams in English Premier league on 19.12.2007:

Class A: Arsenal (40pts/17gms=2,35), Manchester United (2,29), Chelsea (2,00)
Class B: Manchester City (1,94), Liverpool (1,87) ,...
Class D: Newcastle (1,47) ,...
Class G: Derby (0,35)

On 23.12.2007 we had game between Newcastle (class D) and Derby (class G)

Estimations for this game (based on matches between teams from class D and class G in last 10 years in English Premier league) would be:
78% - 12% - 10%

We will go a bit further too. As ratio from Newcastle is 1,47 and is very close to border with class C, we will include a bit of class C vs class G estimations into our final estimations.

Our final estimations would be:
80% - 11% - 9%