When, geometrically, do you know you have BI?

[referee99]

Standard sizes (high school) all the way around.

Saw this tonight. A1 drives in, puts ball off of the backboard, defender pins ball to backboard, just inside the smaller painted rectangle above the rim.

No call from the officials, but I'm thinking that their has to be a line within that rectangle where if the ball is touching the backboard inside of that boundary that part of the ball has to be over the cylinder.

Is there a rule of thumb for this?

Here is a crappy image showing the rectangle, er, "shooter's square".