There are pics of the red curvy lines here: http://lotro-wiki.com/index.php/Threat
But the tank should tell you when he's confident he has enough threat for you to unleash, that's what good tanks do! :-) Some of the strongest guard threat skills are locked behind block reactives, so just waiting isn't enough, only the guard will know. Also be sure you have the Target Assist window open and are following the appropriate target, as the tank will build up more on the primary target, then when it's about to drop, shift on to the next target to begin building threat before DPS gets on that one.
Another key thing, if you screw up, don't keep shooting/building MORE threat on yourself, stop and just autoattack (or even turn those off) so the tank can get it back readily. Also, if you aren't standing right next to the tank, run to the tank (who is probably running after the MOBs).
Note also, once you pull from a tank, they have limited skills to regain threat, and won't be getting reactives anymore for the stronger skills. So give it an extra moment after they have gotten the MOB back again.
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