paycheck and a number of other inputs like my 401k contributions and IRA contributions. I pick my withholding value to withhold just a little bit beyond what my expected tax liability is going to be. So there are really no surprises whatsoever for me. If I make an adjustment to my retirement withholding, I know exactly how it will affect my taxes and I can adjust my withholding appropriately.
Unfortunately 2017 was an anomaly and I made a lot more due to a relocation package, so it isn't really comparable to 2018 at all.