Good overview by Acidfly. I made the mistake of waiting quite a while before starting CM's because of my experience with one try I did when I even still had a blue. In reality, they're tough, but you can do them sooner than you think. Lincoln Tunnel Checkpoint is the easiest if you make use of all the high spots. Once you are able to clear a CM, the item grind speeds up until you hit the DZ50/Tech grind phase, so don't delay giving it a try and don't wait for imaginary milestones. The DPS figure, for example, is really dodgy and only provides a very rough estimate of your power as far as I've experienced. It doesn't take into account some item-based talents, and highly values the "% accuracy increase" stat (which, in a free-aiming game, is completely unrepresentative of reality most of the time). When I cleared LTC for the first time, I had 135K DPS and my friend even just 100k DPS. However, he's a pretty crack shot and we managed just fine. I'd say that as soon as you have a yellow weapon, regularly start trying LTC until you make it.