I agree, you have to just multiply.
But your math is an attempt to abstract human harm to numbers, and the problem I have (and I suspect others intuitively have) is that the abstraction is wrong. You’ve failed to understand the lessons of the science of happiness: we forget small painful things quickly. The cost of a speck in the eye, let us imagine, is 1 unit of harm. But that’s only true in the moment the speck hits the eye. In the hour the speck hits the eye, because of the human ability to ignore or forget small pains, the extended cost is 0. This is why a googol of specks is better than torture, because a googol 0s...
The real problem is defining the boundary between momentary (transient) harm and extendable harm. This is a psychological math problem.
I think this is fundamentally my issue. I think that even when we disregard all differences between people thus making all their personal qualities like pain tolerance identical. There are still distinct lines where inherently a given amount of pain has direct consequences that any smaller amount of pain cannot possibly have inherently, so no dust caused car crashes but the scars of torture count. Meaning that yes, I think, there is some shift where an infinite amount of pain with no inherent long term consistencies, a dust speck, is better than a case with inherent long term consequences, all else equal. Unfortunately in real life this is not normally the case. For example all of those millions of stubbed toes, or whatever, alter your behavior in a small way that is likely on average to be negative. Thus leading to non inherent losses when for example a stubbed toe causes you to miss the elevator and thus be late and thus lose a job. So that all the secondary problems can over a large sample lead to as much, or more, long term harm over the population, but all of that is outside the scope of the problem as presented. And I think makes this a poor question for deciding real actions and a difficult problem to discus clearly.