How do you calculate the drill diameter, given the ball diameter on a PBGA?
You have a major misconception.  You don't drill holes for BGA pins.  You put small pads on the board for the BGA pins to sit on.  In the manufacturing process, solder paste is put on these pads, then the whole thing heated.  This results in the solder making both a electrical and mechanical bond between the pads and the pins.  There are no holes in the board envolved in this process.