federal tax withholding
my wife and I are retired. federal tax is withheld from both of our pensions and from only my social security; no withholding from my wife's social security benefit that began in January 2018. our income in 2018 will be increased over 2017 income by only my wife's SS benefit. we received a small tax refund for 2017 taxes. how can I estimate 2018 taxes due to added SS benefit and new tax law to true-up the withholding?