When doing subjective test such as this, as opposed to plotting as in WBZS and WBM, I often do √2 * Std_time for upper and half that for lower.
So if the time is you would do is 11 than 8 and 16 would be the low and high, or if 5.5 was norm then 4 and 8. This is a progression we have sen before.
This works well for me as often normal is is about 5.5 minutes, and 2:45 is just an unworkable time in the developer. Too many other things go wrong that short.