Usually I blow a bubble about 2x the diameter of the tube then put decoration on the bottom half and condense. I try to keep an area in front of the face blown out larger than the tube diameter so that disk won't run into the tube and trap air. When I'm ready to separate I punty on the bottom of the disk then heat the tubing on the other side of it and blow it out a bit to get it away from the face of the disk if needed. Then I heat that area good and hot and pull it away from the disk while blowing, then pop a small hole and flame cut off the tube. I use a small flame to heat the edge of the thin glass remaining and use a rod to wipe it up and out away from the face until I have a consistent edge. Whether this is the best approach or not is debatable but it avoids the air trap on the face.
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