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For now this is determined on the number of activity attained - which also affects the number of tokens you will be awarded, as well as the total number of contributions to be rewarded.

And I guess there is a maximum upvote value?
As I saw a lot of times 8.93% VP the other day.

To bad I still have to wait for the IPhone version.

the upvote value is actually distributed among participants to be able to cover all contributions, and according to their number of activity per day, which is why with more participants the percentage gets lower.
The iPhone version will hopefully be here within this month :)