Ref the defense - then what?

[Rufus]

Had a situation during a private school VB contest last night, two-man crew. A1 is driving on a fast break, B2 is ahead of A1 and turns to try and establish LGP. My focus shifts to him because I want to know when/if he establishes.

A1 jumps for a try just below the free throw line extended and plows into B2. Because I was concentrating on B2 I knew he hadn't established LGP before A1 got airborne on the try so I called a block.

Here's the question - A1's take off was high in the key. B2 attempted to establish LGP low in the post. There's quite a bit of distance between them so, if I'm refereeing the defense, what can/should you do to be certain of what A1 did, and when, in terms of beginning the habitual shooting motion? I knew/saw in this case that A1 was airborne before B2 got established, but in thinking about it afterward I realized it was more happenstance than conscious thought/awareness and that kind of bugged me.

On a related note I'm starting to understand/appreciate the no-charge arcs in the NBA/NCAA.