Ultimately, you're going to want level 11 (feet, eyes 12). Any lower and you encounter lava pools in front of you, any higher and the diamond rate in the layers exposed drops greatly. Max coverage is to make one long branch then make branches with 2 blocks between them, but I've heard that 3 blocks accomplishes the same results with more ground covered, with the chance of missing a few veins here and there. Your choice, though I go with every 2.
There are no more diamonds at level 12 than there are anywhere else (within the rest of the diamond layers that is, of course.) Diamond ore is roughly evenly distributed between layers 5-12, inclusive. There are slightly fewer to be found in layers 4 and 13, and after that it drops off sharply in both directions. (See diagram below, from the wiki.) The only reason people recommend 11 / 12.62 is because this puts the lava lakes even with the ground you are walking on (not eye level, like you said it was), which makes for a simple matter of solidifying it with a water bucket, after which you can walk safely across your nice, new obsidian floor. Lava? What lava?