That 12 was for Ability Score. I was using same format as you did in quoted part.
In my mind it was:
Ability 12 = +1 Modifier ... Expretise = Prof 2 * 2 = 4 ... Therefore Ability 12 should be enough for character to preform task of Dif. 5 on level 1.
Oh. That's what your meant with 12+! That's just not the proper way to write it, but ok. Well, a rogue with expertise would be fine against 5DC even with 10 point in corresponding ability. Because you can't roll less than 1 and you add your skill and ability modifier to it, so 1+4...
