Originally Posted by bruhaha

Your actual tax rate will depend on your total taxable income when you file your taxes for 2018 in 2019.

What the company withholds depends on how they are going to treat the compensation payment.

It’s covered in supplemental wages section of IRS Pub 15
https://www.irs.gov/pub/irs-pdf/p15.pdf

They can withhold the flat 22% or lump it with your normal wages and calculate the withholding as if they paid you with your normal paycheck which depends on what your filing status is and the number of exemptions you claimed on your w-4.

In addition to the federal income tax withholding Spirit still has to withhold 6.2% for Social security for the first $128400 you make (after you make $128400 spirit stops withholding social security) and 1.45% for Medicare (no limit). After you make more than $200,000 you are subject to an additional Medicare tax of 0.9%.