I am just confused with the flag score?
Why the flag score increase is non-linear after 500? I know there is a max cap score of 750.
Why don't just make it as linear and increase the max cap, say 2000?
Lets start by looking at a graph of the flag weight for 6 imaginary individuals flagging 400 times at various success rates.
(because I don't know the actual algorithm used, I'm going to assume each successful post-flag above 500 will increase the flag weight by
10 * (750-weight) / 250. That should be close enough for our purposes.)
As you can see, anyone with a success rate over 50% will eventually reach 500. Once our imaginary people pass 500, their flag weight starts to plateau at various levels depending on their success rates. If the cost-reward was symmetric then we everyone over 51% would eventually meet up again at the top.
Not only does this allow the system to distinguish between a "good" flagger and a "bad" flagger, but also between a "good" flagger and a "long term ok" flagger.
For simplicity, the graph assumes that people flag consistently with their good and bad flags evenly distributed. In reality, there will be some local variations and some people will change their ways (for better or worse). I found that the flag weight continues to (roughly) track the same path even with small local variations, but a sustained change will cause the flag weight to find a new plateau.
As per Hendrik's recommendation, I revisited this using
10 * Pow(10, -(weight-500) * 0.008) which is reported to be the actual formula used.
The overall result is mostly the same but with more differentiation in the upper regions.
Although its not shown in this graph, the 99% user will still hit the 750 cap in a little under 800 flags past the end of the graph, whereas the 98% user will not reach 716.3. However, I'm not sure such specific details are particularly meaningful in the real world ... except perhaps to determine the minimum number of consecutive successes required between each failure to guarantee some sort of "forward motion" (probably just short of 99).
The idea is that this way your flag weight will always approach a certain number depending on the current success rate of your flagging.
If you're 50% successful, your weight will approach 500. If you've been 100% successful lately it will approach 750. For any rate in between it will asymptotically approach a number between 500 and 750. If your recent success rate changes, it will start adjusting towards the number for your new success rate.
Brian's answer here has some great graphs that explain the concept really well. Have a look at it.
Speculations about the formula are in What's the flag weight formula in the 500-750 range?. The rationale is basically "with greater power comes greater responsibility." Once you're established as a good flagger, another good flag doesn't provide much more information about how good a flagger you are.