Its VERY simple.
Stand where you want, but if someone comes along and wants 'their' seat, you move. That's the price of the loose seating arrangement, somewhere between a free for all and fixed seating. There are plenty of people, including me, who stand at the same place every week.
I have had the same seat since S1, I always stand there, generally with the same people. If there are people in my seat when I get there, but there's room on the back row near where I usually stand, like this weeks game, I stand nearby and let it slide. However, there have been times, when its raining usually, that people large groups have tried to take the space we use every week, they get told to move on, and yes, sometimes they object, but why should I not be able to stand in the same place I've stood (and where everyone who knows me knows to find me) because some random wants to stand there this week.