$660 / 0.085 = $7765, which exceeds the annual contribution limits for a Roth IRA.

The lowest tax brackets are 0% and 10%. If your Roth contribution is being taxed at 10% or less, I'd keep it where it is. I'm assuming it's very unlikely to see that tax bracket in retirement, or at least unlikely to see a better tax rate in the future.

What amount did you contribute to a Roth IRA for 2018?