Check here. You've got the right idea, but a couple concepts confused with each other. Reaction time and running time are separate, but ET is not the same. ET is reaction time plus the time the vehicle was actually running. It is fairly simple from an engineering perspective to measure actual run time and ET, and subtract the two and find out the reaction time.
Well, take it up with the NHRA. It's straight from their website, and I linked it. I have no interest in debating or interpreting or defending their rules or policies.