Shaco deals 20% bonus damage when striking a unit from behind with his basic attacks or abilities.
But what exactly does behind mean?
Is it 10/90/180 degrees?
It's hard to find an exact number without having to look at the game files but from personal experience it's around 180°.
It's probably somewhere between 160° and 180° if you want to put it into numbers. I kind of often play him mid and as a melee champion without waveclear mid you'll find yourself under the turret almost all the time and there you just have to go slightly behind the minions to trigger the bonus damage.
Also in this video you can see around 3:15 that he stands next to a minion and still gets the backstab bonus.