Can I plug in a really large deduction amount on my w4 to balance out the taxes on a 100k bonus that gets added to my regular sm wages?

[Opus 17 · Accepted solution]

Yes, but if you get it wrong and end up under-withheld, for the quarter or the year, you will pay a penalty.  Also, the employer has two methods to deduct taxes on the bonus, one of which methods will result, in your case, in a significant under-withholding situation. 

What I suggest is to wait for the bonus to be paid, see what tax is withheld, then use the IRS withholding calculator to set your withholding for the rest of the year. https://www.irs.gov/individuals/tax-withholding-estimator