Hy!

How can I calculate, or where can I find the method for the probability of an item in a container. I get the weight of an item, then calculate weight / totalWeight in the container. These calculations does not met the ones shown on rustlabs.com.

Do you guys know the real or better way to calculate the probability?

Rustlabs probably pulls the numbers from the code itself.  If the results in practise don't match, that's just how RNG rolls.