My hair grows in different directions in back just below the crown. I have to shave several different ways to feel smooth. And strangely, the randomness of the growth is more pronounced on the left side than the right.
Congratulations on being near the half-way mark. You'r probably noticing that your scalp is becoming accustomed to being shaved and that you are getting better at it. I tried using a mirror for the back, but found it to be awkward, so I do it completely by feel (left hand keeps place and feels for stubble). Give it a try if you feel comfortable.
The 30 day rule really works. It gives you time to adjust. I'm 12 weeks in and my scalp has toughened up (no bumps or razor burn), so the sly look is even better.