I was thrilled to follow your discussion and see where a simple relation related question can take you guys! No blame for hijacking, though!
So cutting a long story short, you strongly suggest to not rely on a calculated value in the relation, especially in regard of speed issues, but recommend to have a script set (and un-set) a flag and build the relation on that.
So, I think I will go for that and say goodbye to my "floating" calculated system. In the end, it is very true what Ugo said: In the real world, things might easily not add up as maths would have expect you, and you might