FED - When to call pitch illegal?

[blueump]

FED rules last night. Runners on. Pitcher steps onto the rubber with his hands already together in the set position. He never started with the ball at his side or behind his back as required by rule. I know this is the beginning of an illegal pitch (balk) but since a balk in FED is an immediate dead ball situation, when do I call it? As soon as he steps on the rubber? When he separates his hands to pitch? Or on the pitch delivery?

I know this is a moot point sonce its gonna stop any play anyway, I just want to be sure that I call it at the right time...not too soon, not too late.