@frenzed If the goal is exact PR then why even have a threshold. I thought we did not want exact PR.
But more on topic I think you are making 2 assumptions you do not know you are making. You assume at all existing parties are independent and uncorrelated. More mathematically, the assumption is that the parties form an orthogonal basis for a political space. This is quite clearly false and I hope I do not need to explain why.
Secondly, by having all of a voters endorsement put towards one party you are preventing exact PR. In fact I would think that in general this effect is larger than the effect you are brining up here. To do this properly you would need to know the vector representation of each voter in the space defined by the parties. Having that you could calculated exact PR.