How to start Action only when Loop finished?

How can I read current looping Value? From {SELECTED ORDERS} I have Order ID# and comma separate value. I can get last value but how can I get current value to check if this the last loop so, I can constraint action not to execute before last loop.
I tried ticket tag but I think write tag and read back is slower than looping. So, value from ticket tag is not correct.

Explain what your trying to do.