Queue Alert has a new option: After getting a rate calculation that you are happy with, you've always been able to switch to the estimated time of completion (i.e., the third option in prediction) to start a countdown. Now, a small chime will play when the countdown ends (unless you've muted the app or your phone). More importantly, you may now add samples during the countdown if you wish. To make this possible, samples will be added automatically when queue members are released from the line based on the established rate. If you add new samples, those will modifiy the rate and update the time estimates.
To do this, check the second option on the "i" tab:
Allowing samples during countdown is off by default.