To calculate perception time, the timeframe in which a character was able to process some information or perform some kind of mental reaction needs to be estimated.
The most common way of doing so requires distance and speed components. The
two distance
s being
the distance an object has when it is first in the perception of the character, and then the distance where the character starts to react. The speed can be something calculated or assumed, but preferably it should be a reliable statement stating how fast a projectile, object, or even another character is.
Be careful to avoid calc stacking, though. To find perception time, you divide the distance (in meters) by the speed (in meters per second), and you will get the timeframe of the perceptions of the character (
in seconds).