When you dig in a straight line, you're exposing 4 layers - your feet/eyes where you're digging and also below your feet and your head. Diamonds are mostly found at layers 5-12 and a sharp decline in appearance begins at layer 13. Therefore, if you want to be efficient then exposing layer 13 isn't worth it.
The 2 best locations to dig are 7 or 11 (eye level). Either of these expose 4 blocks around you that have very high chances of containing diamonds.
There are several levels that will yield the same average number of diamonds; level 10 is one of these. However, level 10 also has the most redstone, and even if you don't use redstone, the experience it's worth makes level 10 the best level to mine on in my opinion. To counter the lava pools you might run into, use water and blocks to turn it to obsidian, and try to get a diamond pickaxe with a high level of efficiency. Efficiency IV will go through it fairly fast.
I also recommend either using silk touch on diamonds and redstone in order to store the ores, then be able to mine them with Fortune III later on when you need the items/experience.