The equation should have per-subject weights to modulate the "how many people" part. For instance, if my app helps Mr. Musk be 10% more productive, that's more impactful than helping Mr. Smith be 10% more productive.
Assuming that a 10% increase in productivity causes a 10% increase in the impact on people (either in quality or quantity), 10% of Mr. Musk's impact may be greater in absolute terms than 10% of Mr. Smith's.
One may debate whether 10% productivity = 10% impact, but if that increase is attributed back to you, there's no need to artificially manipulate the equation.