I have been building AE shocks for the past 20 or so years. The easiest way is practice.
What works for me is to fill the shock and cycle the piston a couple of times to get the air out from under it. Let it the shock stand up right until the air rises to the top. When the oil looks clear (may take a couple of minutes) top off whats left until its to the top.
This is where it works for me. Take a flat metal blade, like a pocket knife, and skim off the oil that is bubbled up over the top of the shock body, then put the cap on.
Its not rocket science, the way I do it, but they are identical almost every time, and close to what the instructions tell you to do rebound wise.
Some times at the track, a blown shock or a quick fluid change is necessary, so its was a good idea for me to find a way that works well, and that is consistant.
I hope it helps.