I use two screens with changa and it works great.
I load the dmt freebase directly on the pad. I then heat the crystals slightly,by lightly hitting the pipe with a lighter to melt the dmt into the pad. When the pad has absorbed the dmt, the pipe is ready to send you to hyperspace. I have been able to get there in one huge hit using this method.
The trick is to have the pipe warm. What I found is that if I just hit the pipe cold, my breath would run out during inhalation, only consuming a small amount of dmt vapor on that first hit. Then I would have to take a second. If the pipe is warmed slightly I found I would begin to see vapor almost immediately after heating with a lighter, thus consuming pretty much all the dmt in one hit.
I hope I explained that ok. Just takes a little fine tuning at first. Once you get a good technique it's free sailing.
"In the universe there is an immeasurable, indescribable force which shamans call intent, and absolutely everything that exists in the entire cosmos is attached to intent by a connecting link." ~Carlos Castaneda