Equally doesn't seem intuitively fair; per-visit is good but has to be adjusted in the face of AJAXy sites and how hilariously easy it is to stuff the ballot box.

I'm going to guess proportionally. Every other such scheme has been proportional. Google can solve the fake visit problem with pattern-matching wizardry. Readability tried to solve it by turning themselves into a proxy server.

I solved it by using a cryptographically-assured end-to-end scheme that requires two mutually distrustful parties to verify that a request was made.