"You work 215 hours per month on average (50 * 4.3 weeks) and make $4,300 revenue during that time (or $51,600/year). - Subtract self-employment tax 15.3% - ~$7900 - Taxable income 37,650 - ~$5,300 in taxes so subtract that"

You don't pay self employment tax on revenue you pay it on profit. So if your paying ex:10,000 in expences for gas then you would pay (51,600 - 10,000) * 15.3% = 6364$ in self employment taxes.