Fine tuning is more of a process of trial and error but in general if you're trying to radiate the signal out (a 1-story house for example) you want to leave the antennas vertical. If you're trying to radiate out and up, like in a 2-story house, you'll want to position at least one antenna at a 45-degree angle or so.
For both the Netgear R7000 and Asus AC68U, I found my best coverage was with the middle antenna at 90 degrees forward/back as well as left/right (aka straight up and down). The two outside antennas I had positioned back 45 degrees and out 45 degrees, so that the antennas essentially formed a "W" shape kind of like this:
front view: \ | /
side view: |/