<prompt> Human: Ive finally latched on to the idea of forfeiting a big refund check in favor changing my withholding to accurately gauge my taxes; and using that net difference to invest monthly into my 403b or start a Roth. - 34 year old in Georgia, file single- made 81k in 2017, contributing 10 to retirement via work 403b- on PSLF so getting my AGI down is importantWhats the best way to go about calculating this difference? Should I just let my first full pay period in 2018 stay as is at 0 withholding, then change it to 1 in my second full pay period? If I do it this way, I figure Roth for 2017 is valid option. I can fund it interdependently, right? vs. being withdraw from paycheckOther pointers I should be aware of in making this change? Thanks.  <response>: Use the IRS withholding calculator when it updates for 2018. 