The theory is that, while generating scores with 3d6 rolls, the average score will be 9-12 (this is also the range that gives you no bonuses or penalties). Then, assuming that adventurers are somewhat tougher than the average person, having 12 in all 6 stats is considered "normal". This adds up to 78 points to distribute between ability scores.
The next thing to take into consideration, is the ability requirements. e.g. a required ability score of 14 would give you 1 extra point to assign. This adds up to the following scores for all classes:
Barbarian: 78
Bard: 82
Cleric: 78
Druid: 81
Fighter: 78
Mage: 78
Monk: 78
Paladin: 83
Ranger: 84
Sorcerer: 78
Thief: 78
Finally, there are some extra points to be assigned: a dual- or multiclass character gets 3 extra points, while a protagonist gets another 3 points to reflect his godly nature.
I've used this method ever since BG1, in both single and multi-player parties, and found the game quite challenging. Not having all 18's can be fun
Also, this makes characters such as the Kensai/Mage or Kensai/Thief a bit harder to create
What do you think?
What are your score totals?
TC, Geert