Hey there!
Okay. After wrestling with the books last night it appears that instead of 13/16 + 1/2 level for the main stats that, for epic level monsters anyway, something akin to...
19 + 1/2 level = Best score
16 + 1/2 level = Second best score
13 + 1/2 level = Third best score
...works best.
I think if you wanted to mix and match you could have
3 Ability Scores = 16 + 1/2 level
(other three 10 + 1/2 level or less)
or even...
6 Ability Scores = 13 + 1/2 level
Example of first method:
Orcus: L33 (Top 3 Ability Scores)
19 + 16 = 35
16 + 16 = 32
13 + 16 = 29
There are two (epic) monsters in the book that notably break this, the Tarrasque and the Godforged Colossus.
Frankly, I am not happy with the Tarrasque. Even going by their own monster creation rules in the DMG it should be dealing 4d8+12 instead of 1d12+16. I mean I know it gets the +15 ongoing damage but it should have that on top of the higher base damage.
By comparison the Godforged Colossus deals 4d12+15 with its basic attack! Its actually smaller than the Tarrasque, weaker, lower level and only Elite where the Tarrasque is a Solo monster. Yet its dealing on average 18 points more with a basic attack. Before you say, well what about the Tarrsque's +15 ongoing damage. Well the Godforged Colossus gets a secondary attack that deals 3d6 psychic damage and stuns you.