How can this be explained mathematically?

Umm Fiel, the reason why those values are off is because pddRounded is rounded

Well, at least I got that the error was in rounding, just on the wrong value!

Thanks for the proof.

And for your calculator, you can't use this shortcut because the % curve is not continuous, discrete rather since % reduction is floored

I went into this topic thinking it was not floored, and now I know that it is floored after this thread. It was because I thought it was not floored that I thought this heuristic would work.

Also, it's not % * 100. 70% is 0.70 in decimals, so multiplying that by 100 would only give you 70 and not the 7000 required.