According to Kaplan, the following is a breakdown of what consititutes a 20 on the DAT, broken down into sections:
Biology - 34/40
Gchem - 26/30
Ochem - 25/30
Total Science - 81-85/100
QR - 30/40
RC - 38/50
PAT - 70-73/100
Like savvy said, I believe these are estimates, but they are pretty close to the real thing. To score 20 is dang good because after you miss 1 question on each section, your score drops from 30 to 26 or 27. So getting a 20 means that you missed very few, obviously.