This is a question regarding infinite loops and shortcuts at Competitive REL.
Here is a potential scenario.
Both my opponent and myself are at 20 life each.
I have no creatures on the battlefield.
I have a Rakdos Charm in hand.
My opponent plays Deceiver Exarch and begins a loop where he uses Kiki-Jiki to copy the Exarch, and uses each new Exarch to untap Kiki-Jiki, dealing me 1 damage every time he does it.
If I cast Rakdos Charm when the 18th Exarch token's trigger is on the stack, I win.
If I cast it at any other time, I lose.
If my opponent knows that I'm going to cast it than he won't make the 18th token, and he'll simply attack and kill me.
Now, the question is:
If my opponent proposes a shortcut, saying
"I'll make 500 tokens and kill you."
Can I accept the first 18 iterations of that shortcut and react by saying
"After the 18th token enters the battlefield and its ETB trigger goes onto the stack, I cast Rakdos Charm."
, or do I have to propose a new shortcut for the 18 iterations, potentially giving my opponent information?