How much is the tax on a 350.00 oz of product

Multiply $350 by 0.2 then add the result to $350. That will give you the final cost of the oz including the 20% tax.

Are you asking someone to do math for you? "Gov. Charlie Baker in July 2017 signed legislation increasing the maximum combined state and local tax rate on recreational marijuana to 20 percent. Under the law, consumers will pay a 10.75 percent excise tax in addition to the state's regular 6.25 percent sales tax. Cities and towns would also have the option of adding a 3 percent local tax." So add 20-25%