"In real-world encounters, many variables affect time, which is the key component of the 21-Foot Rule. What is the training skill and stress level of the officer? How fast and agile is he? How alert is he to preliminary cues to aggressive movement? How agile and fast is the suspect? Is he drunk and stumbling, or a young guy in a ninja outfit ready to rock and roll? [...continues for a while...]"
I'm not sure what point you are trying to make. In our example, none of these questions about the suspect are known. So the officers have to assume that this attacker can (and in fact, he was in the process of).
"In real-world encounters, many variables affect time, which is the key component of the 21-Foot Rule. What is the training skill and stress level of the officer? How fast and agile is he? How alert is he to preliminary cues to aggressive movement? How agile and fast is the suspect? Is he drunk and stumbling, or a young guy in a ninja outfit ready to rock and roll? [...continues for a while...]"