Melee AC needs 110% melee accuracy. Force-Range AC needs 110% force-accuracy etc. In my case, Sorc dps, 110% equals about 400 accuracy rating. Even with 109,9% there is a chance to miss. Some ppl prefer 108% to pump a bit more into power but it's luck based and for some classes like the sorc it's a massive dps loss if one of your main abilitys miss in certain situations. Same with a Tank, if you're beneath 110% your aggro-spells could miss wich could cause wipes in certain fights.
However i am not sure if the rating is the same for every AC. Not sure if you have to have 400 or less Accuracy rating on your gear with tech base classes etc. I just don't know that because i don't play a tech based class. Just stick with 110% and you're good to go.
Mostly correct, but felt I should jump in and correct the mistakes
Bosses do not have any defense rating, but all elite mobs have 5% resist chance to force/tech dmg and champion mobs (like bosses) have a 10% resist chance to Force/Tech dmg.
Melee and Ranged attacks have a base of 90% accuracy. This means you should get 10% more if you want to hit every single time.
Force and Tech attacks have a base of 100% accuracy. Therefore you need none against mobs without a resist chance. For Elite rating mobs you should have 5% more in Force/Tech and for champion mobs (which is what you gear to beat in operations) you should get 10% more accuracy to hit every single time.
As the quoted player wrote: Some people prefer to leave their accuracy a percent or two behind the "hit-every-single-attack" mark to stack more power. You can consult theorycrafters on this forum and oter forums to see if that would work for your spec
When it comes to tanks: Taunts NEVER miss. Even with 90% to hit with an attacka tank should easily be able to maintain threat, and can use taunts to boost it in most fights IF he needs it. Everyone can get 1% accuracy for completing all the conversations with a melee tank companion and some can get more from their spec (3% for assassin tanks). A tank should focus on mitigation stats and no good tank has ever swapped in accuracy modifications instead of mitigation (defense, absorb and shield. Some would also include endurance, depending on which type of game they have tanked in before).
Hope this helps