For the playing card method, it's actually pretty easy (unless my math is really screwed up here.)

The cards are whole numbers so we can't get an average of 12.5, but let's say average of 12 across the six scores.

Six scores at 12 points each is a total of 72 points.

Now we will do decks of 2 cards per ability score, which is a total of 12 cards. We need the numbers of all 12 cards added together to equal 72 (so when you divide it by 6 it will equal 12.) You want the possibility of an 8 and a 15, which means you need at least two '4's, and can have only one '8' (two or more could give the possibility of a 16 or higher.) So a card distribution could be this:

two 4s (8 points)

two 5s (10 points)

three 6s (18 points)

four 7s (28 points)

one 8 (8 points)

8+10+18+28+8=72

Now when a player wants to create scores, they shuffle the 12 cards and deal out facedown 6 piles of 2 cards, flip them over and add them together. They will have 6 scores that in total will average 12. Usually to create more interesting characters when I do this method I make the players use the scores in order, which makes them have to come up with classes they ordinarily might not get to play because their highest score was in an ability they wouldn't ordinarily have chosen. It also will occasionally give you PCs that won't have a CON of 14 or higher, which almost always seems to happen when you go with Point Buy. No one ever buys a low CON. This method means they occasionally might.

If I've screwed up or misunderstood what you mean by 'an average of 12 or 12.5', then this method wouldn't necessarily work. But if I got your 12s across six scores right, then in theory this should work.

You just convinced me to use this method with the Tarroka deck from GF9 for my next campaign.

0) The players pick a race

1) I'll pick 6 cards representing each of the 6 Ability that the player will need to draw, then align in order, face down, before drawing their Score cards piles.

2) The player draws their Score cards, face down and put them in a Celtic cross spread.

3) The player picks one Ability card, turns it face up and select one pile from the spread.

3) The player turns their score cards and notes which Ability got which Score.

4) The player picks a class that complement its choice of race and stats they got.

EDIT: Just went through my Tarokka deck, and here's the card used for this chargen method:

**Ability Card**
Marionette: Dexterity

Raven: Intelligence

Warrior: Strength

Beast: Constitution

Seer: Wisdom

Tempter: Charisma

2 x 4 points: Shepherd, Abjurer

2 x 5 points: Druid, Guild Member

3 x 6 Points: Evoker, Anarchist, Beggar

4 x 7 points: Charlatan, Hooded one, Illusionist, Thief

1 x 8 points: Bishop