It is not as simple as a percent of your sales. Your self-employment tax will be 15.3% of your net earnings. That consists of 12.4% Social Security and 2.9% Medicare tax. It does not include your regular federal, state or local taxes.
Your net earnings generally are not the same as your sales. Your net earnings would be your sales income minus selling and other expenses. Therefore, your net earnings are likely to be lower than your total sales.
Your federal tax rate will depend on your tax bracket. State tax rates vary on how they are calculated as do local.
If you use a program like QuickBooks Self-Employed to track your income and expenses the program will help you figure out your net income and quarterly taxes due.
Also, don't forget you should be making quarterly tax payments, not just one payment at the end of the year.
https://ttlc.intuit.com/questions/2926899-how-does-my-side-job-affect-my-taxes
https://ttlc.intuit.com/community/self-employed/help/what-is-the-self-employment-tax/00/25922
https://ttlc.intuit.com/questions/2902389-why-am-i-paying-self-employment-tax
https://ttlc.intuit.com/questions/1901340-where-do-i-enter-schedule-c
https://ttlc.intuit.com/questions/3398950-what-self-employed-expenses-can-i-deduct
https://ttlc.intuit.com/questions/1901110-do-i-need-to-make-estimated-tax-payments-to-the-irs
https://www.calcxml.com/calculators/self-employment-tax-calculator (non-TT site)
I've been self-employed for 16 plus years now. What I've done since day one, is to send the IRS 20% of my "gross" business earnings each quarter. Then come tax filing time I'm always well within $1000 of my tax liability. Usually, I get some of that money refunded at tax filing time. But the two times I owed the IRS, it was less than $1000. Therefore, no underpayment penalties were assessed. This method has worked just fine for me.
Now I don't have to deal with state taxes. So if your state taxes personal income you should send your state a quarterly payment equal to whatever percentage your state taxes personal income at.