The angle of the dot is the angle at which the dots chain together. The problem with most computer graphics programs is that the angles of the halftones are generally great for offset printing but not as effective for silk screen printing.
Most computer programs use 45 degrees as the default angle. Actually, 20 to 25 degrees is good for basic halftone work. If you are doing a process color job you can try Cyan 15, Magenta 45, Yellow and Black 75, or Cyan 22.5, Magenta 52.5, Yellow and Black 82.5.