Stop the clock when it feels like the target has passed, without counting. Three rounds at 10, 7 and 14 seconds give a mean absolute error for your sense of time.
The targets deliberately do not ascend. A regular progression lets you scale your first-round feel arithmetically, which measures multiplication rather than time sense.
Stopping late means time felt slower than it ran; stopping early means the opposite.
Counting is the same as looking at a clock and the result stops meaning anything. The figure is only worth reading if you went by feel.
With more rounds you start correcting from the previous round's feedback, and the test quietly shifts from measuring time sense to measuring calibration.
The grade bands on the result screen show the ranges. They are reference bands, not norms from a validated instrument.