The most frequent upvoter sets the target for 100% (in the example, this was 25 upvotes over a 28 day period). Your bonus is your vote frequency as a percentage of target, times your average upvote weight.

Let's say you get up to 25 upvotes, but some of the upvotes for the person with 25 drop off (older than 28 days), so you are now setting the bar. You would get 100% for frequency. If you always upvote at 10%, you would get 100% for frequency times 10% for your average upvote, or a 10% bonus.

So in the original example, the second member upvoted 20 times at 90%. Their frequency was 80% of the target, and their average upvote was 90%, so the total bonus was 72% (0.8 * 0.9)

I hope this helps!