What percent should I have withheld from my social security check for tax purposes

Go to this IRS website to read about the tax withholding estimator and a link for same - https://www.irs.gov/newsroom/tax-withholding-estimator-helps-retirees-figures-tax-on-social-security-benefits

Social Security rules state that you can have 7, 10, 12 or 22 percent of your monthly benefit withheld for taxes.  Only these percentages can be withheld. Flat dollar amounts are not accepted.