Hi there! To answer your question, Step 2 line (a) yields the most accurate result, per the IRS - as stated on page 2 of the form instructions (see below):
"Step 2. Use this step if you (1) have more than one job at the same time, or (2) are married filing jointly and you and your spouse both work. Option (a) most accurately calculates the additional tax you need to have withheld, while option (b) does so with a little less accuracy. Instead, if you (and your spouse) have a total of only two jobs, you may check the box in option (c). The box must also be checked on the Form W-4 for the other job. If the box is checked, the standard deduction and tax brackets will be cut in half for each job to calculate withholding. This option is accurate for jobs with similar pay; otherwise, more tax than necessary may be withheld, and this extra amount will be larger the greater the difference in pay is between the two jobs."
Since they actually state that (a) is most accurate, then I'd go with that. Many people don't want to use the calculators or don't have all the info handy to fill in for the calculator to work correctly and just choose (c) since its easy! But if you are looking to be the most accurate, (a) should be best. Again, once the changes are made, take a look at your paystubs and make sure you are withholding enough, regardless of which line you choose in Step 2. That will really be the only way to tell for sure.... (Remember, this year is almost half gone already, so you may need to redo this exercise for the new year. This holds true even if you don't change it because you don't mind more being withheld for 2025 as there may be life changes that would impact like a change of jobs, one or both gets a raise, one or both gets a large bonus, dependents, etc. Just keep that in mind.)
