Note: I tried to get a proof of this with my balance and a calculator but didn't got the same results maybe I'm missing something or using the calculator wrong (like most of the time) so better wait to someone else with better maths to confirm if it is like i wrote it.
You got all the details right:
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Here's how the calculation works. Divide your onsite amount by the total onsite amount, multiply by 0.9 and multiply by the stake amount. That will give you your share of the stake reward. For example:
Thank you dooglus your formula works like a charm. the approach that was taking was way more longer and results didn't was not even close to be right so that's was why i didn't posted it. I was trying to apply a rule of 3 to know what percentage of the onsite bankroll represented my onsite coins, then tried to know how many clam was that on a given stake message and see if it matched with the "your share" part of the stake message. At that point my result was super wrong so I didn't even bother to deduct the -10% commission.
The moral of this: use dooglus' formula. lol