Police Commissioner Mulrooney today announced a new police precinct at 70-01 and 70-03 Grand Ave., Maspeth, will be opened at 4 p.m. tomorrow and will be known as the 112th Precinct. Included in the precinct, which will have a force of 110 patrolmen, are Middle Village, Maspeth, part of Winfield, South Elmhurst, Ridgewood Plateau, Rego Park, Forest Hills South and Forest Hills Gardens. Police Capt. Peter McGirk, formerly at Far Rockaway, will be in command.