The Tax Cuts & Jobs Act has brought changes in tax law, many of which may have impacted your withholding calculations. The IRS is still perfecting the Form W4 which helps you and your employer withhold the correct amount from your paycheck. If you discover at tax time that your income tax is now significantly under or over withheld, you may wish to complete a new Form W-4 to adjust your withholding. Forms W4 completed before 2018 may no longer be accurate to your tax situation, so if it's been a while since you completed one, it's a good idea to get it updated now.
In addition, MT Dept of Revenue has released a separate Form MW-4 to calculate state income tax withholding and warns that the federal Form W-4 should no longer be used for this purpose. This means you must now complete two separate withholding forms when you start a new job.
Please advise your employer if you wish to adjust your federal or state income tax withholding for 2019 by completing an updated Form W-4 or MW-4. If you have additional questions, you should contact your accountant or tax professional.