When, geometrically, do you know you have BI?

[referee99]

I'm talking about Basket Interference.
Part of the basketball may have been in the imaginary cylinder with the basket as its base.
Wanting to know where that area is on the back board.

Recently Andris Biedrinch of the Warriors was called for a violation when he knocked the ball off the flange during live action. In NBA it was goaltending (i think) but would have been basket interference in HS.

If the ball is contacting the backboard directly behind the basket part of the basketball MUST be in the cylinder, given the size of the ball.

At some point further away from the center of the backboard this must also be true. How far out can you go and have an automatic, provable by geometry basket interference?