TurboTax w4 Calculator https://turbotax.intuit.com/tax-tools/calculators/w4/ is a great resource to accurately estimate your Federal Withholdings. Yes, you should use the 3-job section of 2b on the W-4 for the most accurate results.
To adjust the states you should refer to the instructions for each state.
SC - https://dor.sc.gov/forms-site/Forms/SCW4_2024.pdf
NC - https://www.ncdor.gov/tax-forms/nc-4-employees-withholding-allowance-certificate/open
The amount withheld will depend on the residency status and the income from each state. If you're splitting income between states or you may have a credit for taxes paid to another state. If you are a resident of SC, all income regardless of where it was earned will be taxable to SC. On the resident return you would receive a Credit for any taxes paid to another state to avoid the double taxation. SC will want a nonresident return if you work there but reside in NC, so you still want to withhold on that income. On the SC W-4 you would calculate it based on the income for that state. The NC you would calculate using all the income since they tax everything.
