When did this become about natures?
The short answer to this question is that there is no easy way to know for sure. If the calculator hasn’t worked, you should level up your Pokémon and track their Stat Exp intake, then enter the numbers in again. This will limit the effect of rounding and give you a more accurate answer.
I wouldn’t spend any more time finding calculators, guys. If one didn’t work, then it’s unlikely another will. The problem likely has to do with OP’s process.