Does anyone know when the RNG to determine whether you get a best piece or not occurs? Immediately after you hit enter or just before the message occurs?

I know it doesn't matter most of the time, but I like having the gift of the gods running when I combine two best pieces (hilt and blade) and at a little over a minute to complete the task it can mean the difference between a lucky second roll making a perfect and not getting that roll.

Starchitin

A severed gnomish hand crawls in on its fingertips and makes a rude gesture before quickly decaying and rotting into dust. A gust of wind quickly scatters the dust.