No the proposition doesn’t make sense. The 3% number comes from this: https://rethinkpriorities.org/research-area/welfare-range-es...

The page gives 3% to shrimp because their lifespan is 3% that of humans. It’s a terrible avenue for this estimate. By the same estimate, giant tortoises are less ethical to kill than humans. The heavens alone can judge you for the war crimes you’d be committing by killing a Turritopsis dohrnii.

Number of neurons is the least-bad objective measurement in my eyes. Arthropods famously have very few neurons, <100k compared to 86b in humans. That’s a 1:1000000 neuron ratio, which feels like a more appropriate ratio for suffering than a lifespan-based ratio, though both are terrible.