How to calculate 50% success threshold?

In ‘Getting Started with Neurofeedback’, the author, Demos explains 50% success threshold:

"… the threshold bar has been set so that reinforcement is received about half of the time. "

I guess I could start with using a timer that measures how long the signal is above and below threshold. Then, depending on the percentage, I could manually move the threshold up or down.

What I am not sure about is how long I need to run the timer. Do I calculate this success in a 10 second time period or longer?

Is this time period the same for all brainwaves e.g. theta and alpha?

Cheers, k