NFPA 1125, the code motors are certified to by NAR, Tripoli and CAR: the burn runs from where the thrust first rises above 5% of its peak to where it falls below it (7.8.3), and the total impulse from that rise to the last measurable thrust before ejection (7.8.5). Absolute counts every newton from liftoff. The same code lets a production lot vary 20% from its mean impulse for a model rocket motor and 10% for high power (7.8.6). Both figures follow the loaded weight: 1% out on the weight is about 1% out on the impulse. Here the thrust passed 5% of its peak before liftoff, while the pad held the rocket, so the NFPA 1125 figures start at liftoff.