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The estimation is somewhat faulty, the important thing to understand is that it ticks up gradually every 2.4 hours until you can receive our targeted minimum upvote value.

but from what I´ve seen, this targeted minimum upvote value changes too.
It said 0.14 yesterday, today it says 0.18.
this is the part I do not get.

That's the part that's faulty. Our actual minimum upvote target is $0.02; we recalibrate the minimum vote target (in rshares) once a week by comparing the estimated value of the minimum votes we're actually delivering to the target value.
When market conditions are more volatile, the accuracy of this estimation decreases.