I actually used a tape measure to scout out my first few trees. My paces gave me some long spreads between trees.
Once I got the approximate distance in my head, the rest was setup and tweaking. Once you do it a dozen times, you can do it in the dark and get pretty close every time.
I've actually gone out and setup my hammock on my deck in the dark and rain, just so I have a basic idea of what I have to fight if I were to do the same on the trail. When I eventually *did* have to setup in the dark *and* rain, I didn't even blink (while everyone else was still struggling with their setups in the dark I was already heating up some food).
As with anything, practice, practice, practice.