Mark,
I’m in the middle of this process right now. I just finished gluing the two rear windows. I’ve drilled and prepped the door windows and windscreen. The Glastar videos suggest a 3/16” gap. That’s the gap I established and it has worked well. I understand your question and believe you could go with a smaller gap. However, there have been a number of locations where a smaller gap would be a bit of a challenge. The issue has to do with the bevel molded into the vertical lip of the fiberglass. You should be able to get a drill bit into a tighter gap but you’ll have to drill into that bevel without damaging the plexi. In any case, I wouldn’t suggest a gap closer than 1/8”. You might give it a try and if it doesn’t work trim more off the plexi and drill new holes.
With regard to the discussion about painting prior to installation. The videos discuss that option and the modified technique for filling the gap. As long as you properly sand the flange you should have no problem with adhesion. The spacers will make it easy to establish a level transition.
From the videos the process seems a bit onerous. Fact is, once you get on a roll, it’s quite easy. I’m very happy with the results.