For years we used the left corner of the room,never had him run away once.Then when I restarted up a static last year,the corner was 50/50.So our THF sggested the blocked door in the middle of the room.We started using that and it had about an 80% success rate,but sometimes even with DoT's on,and in the right place,he just kept running.
Since the last few posts are unclear,I'll just state that the way this rat works,is he isnt supposed to warp away.He's like the rat on the flayer/ghost NM floor.He runs to where you port up,and back to where you find him,and back and forth.Of course if you dont chase him/use the corner/door/wall trick,he'll just idle at the start until you get near him again.