Thank you so much for your detailed response. I appreciate it greatly. The letter streams are presented in distinct trials which are branched together and based on your response, I can see that the "lateness" can be flexible. I was aiming for having the response window open as soon as the stimuli is flashed and have it close after 200-300 ms, which, if I am not mistaken, can be implemented depending on when the systembeep is triggered.
Thank you very much once again, this was very very helpful!