Just a small point for OP - are you talking about a GAI calculated from extended norms? It's probably not material to your question since either way it's >99.9. But if you were somehow trying to get more granularity on that (i.e., is it >99.9 or 99.99 or 99.999, etc.), the distinction might be important.

Or not. I'm really ignorant here. But I do see the note re "extended norms" systematically attached to our DS's scores and discussions about them.