Mrs. Brown has advanced colorectal cancer and has been receiving 1,000 mg of IV morphine per day with fair control. She rates her pain as a 4 on a 0–10 scale. Sedation is a major side effect, and she frequently falls asleep during conversations. She has been admitted to the hospital for a trial of an intrathecal analgesic in anticipation of placement of an implantable intrathecal pump. What is the approximate equianalgesic dose to begin the intrathecal trial?