This past year was goofy. A couple of years ago, they switched to lower paying formula. This year, they switched back to the higher paying one, but the "old" formula counted for the first 9 months of the year. So for all earnings from Jan 1-Sept. 30, it worked out to about 8.5% of my earnings. For the balance of the calendar year, it worked out to about 15% of my earnings for that period.
The current payout thresholds are: 10% of profits for the first $2.5B of profits and 20% of profits in excess of $2.5B.
They take that number, and divide it by the total amount of eligible wages earned by everyone. Whatever that comes out to is the payout percentage. They then take that percentage number of your income, and that's what you get for a check.