Let items pass at a fixed rate from a variable-rate stream

Question

I got a stream of items and for each item an overall rate is updated. How can I decide based on this rate and a constant target rate if an item should pass or be rejected?

I started off using

$$ p_{pass} = \frac{target\_rate}{current\_rate} $$

but this seems to skew the results, as the rate I get out of it is about 60% of the target rate.

Is there a way to determine if an item can pass to achieve the target rate for this variable-rate stream?