I'm assuming you mean you don't want to pay taxes at tax time/want your W2 withholding to equal your total tax liability at end of year? Or do you mean you want to completely eliminate your taxable income [I]entirely[/] via use of deductions and/or refundable credits and receive some portion back? The latter would mean getting your taxable income down to where tax credits eliminate your entire liability and you maximize your available refundable credits.
Precise is going to require information. Leverage what you already entered into your 2018 return software to avoid re-entry.
You can game it yourself using your tax software and saving a copy of your actual return, and tweaking the W2 entry values for #1 Fed taxable Income (the amount adjusted down when your 401k contribution goes up), and #2 your federal tax withholding. If nothing else changes from your current return, that's an easy way of doing it.
IIRC, calculating the change in simulated withholding is pretty straightforward as a percentage of your federal taxable wage for an entire year if you have your pay stubs available, unless bonuses or other W2 oddities are involved. But those latter items can be nearly impossible to estimate with precision anyway since you won't know the basis or the growth for next year.
That's the easiest way to make sure you're including the impact of things like the scaling of non-refundable tax credits or AMT in your estimate.
