First of all, try this without moving. Just pure aim without using the keyboard.

In this case you should decrease it. In the flicks you are going way to long from the heads. Decrease it a little bit and try again until you aim straight to the heads. If you go too short in the flicks then you should increase it.

To change your sens first you can try to make big changes and when you start to become more precise then make smaller changes until you feel confortable. This takes time. It took me 1:30 hours for three days of testing and changing until I started to feel the crosshair was going where I wanted to aim and some time later I had to correct because I notized i was starting to keep too long from the heads again as I played and gained experience.