The best height to mine for Diamonds is y -58/-59. Generally speaking the further down in the world, the diamond distribution grows linearly, but because the downmost layers y -64 to -60 do contain bedrock, potential diamond ore is destroyed. As for tactics, if you go mining by hand I suggest digging 1*1 tunnels at y -58, to maximize the revealed blocks per mined block [at y -59, we would waste some blocks bc the floor can be bedrock]. As for branching your mine, dig into both sides every 3-4 blocks, and try to mine along chunk borders Use water or trapdoors to go into crawling position, and go digging. If you’re more sophisticated, or need a lot more diamonds, I suggest building a honey/slimeblock tunnel bore. There are many tutorials online for simple ones, but I think if you ask nicely, there are a lot of nice people here on Reddit who will give you schematics for some actual monstrosities You will however need a decent amount of ancient debris for them, good thing is, they also work in the nether (though I don’t know what’s the optimal y level for them, optimal for tnt/beds by hand is y 15, so probably a bit lower, so the duped tnt explodes at y 15) Please make sure you play the right version of the game before trying to use slimeblock tnt duping, as many server versions of the game “fix“ them. Test them in creative before you build them in survival, better safe than sorry More on reddit.com

You can try this tutorial https://youtu.be/G87YkNjBToo More on reddit.com

I am often successful between -53 & -55. I also recommend you explore any large openings that you find. I collect more diamonds from those areas than when I am strip mining. More on reddit.com

Someone should do an in game test to get the real values as those values doesn't account for air blocks that could generate. Still, great post for someone like me who likes mining a lot :p More on reddit.com