I think your number, 15.3% is wrong. The percentages vary based on $$earned, if you are paid weekly, bi-weekly, or monthly. And it's based on dollar amounts. Earn $100 and you pay 'x'. Earn $100.01, and you pay 'x' plus something more.So people making anywhere from 90 to 100 would pay the same. 100.01 to 125 all the same.
Before computers, there used to be a booklet called 'Schedule E' which had all of the tax rates. Look at $ earned, based on pay period, and use the number listed.
Also incomes over $107K (?) pay no SS taxes, past the 107K amount.