You need 900 points for Rank 5 ... the thing I did is killed a minimum of 7 people and for everytime I was seen one more. The kill X missions are a lot easier, cause the stealth kill of the boss gives you loads of points.
If you kill an innocent by accident just reload ... saves you time and is faster than weight the minuspoints off with stealth kills.
The point I dont get is ... sometimes I get a bonus for stealth killing everyone and sometimes not in the collect this or find this missions. Even when I´m 100% sure I stealth killed every fucker in this round -.-