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:
1) stake rewards are split in proportion to onsite invested amount
2) rewards are accumulated and given out every 3 minutes
3) 10% is taken off the top of the staking rewards before they are given out, not at the end of the week
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:
