in my experience robb's 75-80% figure is pretty accurate. personally i snatch 80% of what i CJ. i also smoke crack. also just saw this on goHeavy
2) people are split both ways (no pun intended initially, but now i like it). i think the diifference between max clean and max jerk depends greatly on how an individual trains. for example if an athlete cleans way more than he jerks, clearly he needs to focus more on the jerk and less on the clean (and squats). if the disparity doesn't diminish over time, he's probably doing something wrong because there's no point (for a competitive weightlifter) to be significantly better in either the clean or jerk since you don't get a choice of one or the other in a meet. i can jerk more than i can clean, but when God was handing out legs, I thought He said eggs, and i said No thanks, got to watch my cholesterol.
3) You can use the fractionals as pierre said--just be careful not to use them too much and wind up doing way more reps than you really need to. over time you'll become better and better at gauging how close a given effort is to max and be able to pretty accurately choose your next attempt.