I think the idea of "plotting" to the middle of the screenplay has served me well. I had a general idea of where I thought the story would go, I got to the end of the plotting, and now the story has legs enough to take me where it wants.
I think I like the bits that are coming from the non-plotted portion better, too. They are less mechanical for instance.
Okay, I feel a crummy cold coming on (and wrote despite it!), but I'm calling it a night!