If there are more than one mob, KLH reports the overall figure as well unless a master target was set. I think Omen Threat meter can figure out between different mobs if they have different names.
Lets see if I can recall this from memory:
Assuming the tank has aggro, it takes 10% more threat from a melee ranged attacker to pull aggro and 30% more threat from a ranged attacker. Its best to try and keep below the MT at all times so those burst blasts (that will likely put you over) don't steal aggro.
I can't remember the threat modifiers for everyone, but whatever amount of threat one produces from damage is modified by some classes for some situations. ie: def stance for a warrior, 130% standard threat from damage, 145% with talents. Berserker and battle stance get 80% standard threat from damage. I think rogues are 90% (they have threat and aggro drop abilities to counter), but I believe most everyone else is about 100%. I can't remember though. This does not count special abilities that are intended to increase threat.
As Omarr says though, its a ballpark figure since these are all just mods and aren't precise.