At some point percentage is insufficient, but it's because it's a rate. Permillionage/DPM doesn't fix it. It's the number of people affected that matters, so if you have it at 99.9% and grow 10x, you ought to improve it to 99.99% to not become eviler. If you just stay at 99.9% when you grow 10x, you're harming 10x the people.

I'd use the total number of false positives as the proper measure.