… sorry … I read the article on Rebel’s website about Kelly’s formula but I still haven’t figured out how to calculate it … someone could help me: Do you know the exact formula to calculate the Kelly ???

… this is the formula shown on Rebel’s website … so let’s make an example:
if I have a bankroll of € 500 with an odd of 2.3 and the kelly percentage set to 30%, how much stakes will I have to bet ???
according to the proposed formula these should be the steps:

Calculate the Probability of Winning: 100 / 2.3 = 43.47%;

I calculate the probability of losing: 100% -43.47% = 56.53%

now I have all the elements and I can apply the formula: 56.53-46.47 / 2.30 = 4.37;
… so since I have set the kelly to 30% I have to enter a stake of 30% of 4.37% ??? so about 0.9% of my Bankroll ??? so € 5 Stake ???

There’s a mistake.
With the given values the formula should look like this:
(2,30*43,47-56,53)/2,30 = 18,89% , means you should bet 94,5€

The setting for Kelly is what is the maximum % of bank which the software should propose to bet.
Means if the formula suggest to bet 35% of the bank, the software will suggest only 30% of the bank. Let’s say your bank is 100€ and when you calculate the Kelly with the formula the result is 35% (35€) the software will suggest betting 30€, not 35€.

… forgive me but what you write is not the Kelly formula described in Rebel’s disto … and furthermore the result of what you write is not 18.89% … mathematics is not an opinion if you feel like trying to be more precise and list the formula in a precise way and all the steps to get to the result you showed