The only precise way to do it is when you pop P2 the first time to keep going until waves of 3 start coming, then you can easily sort the "stack" position for your initial Champion.
Yeah the minimap does a pretty good job of highlighting the middle of the room you just need to sort of guess where the exact middle is from the given circle. You can use your own minimap icon to figure this out though. Its still not the end of the world if you don't get the dead center of the room.