I would suggest using the IRS withholding calculator to determine what to put on the W-4.
https://www.irs.gov/individuals/tax-withholding-estimator
Alternatively, you might fill out a new W-4 following the new instructions (including the special worksheets for multiple jobs and itemized deductions, if they apply to you) but not adding anything extra. Submit it and see how your withholding changes. After a couple of paychecks, if you still feel you are under-withheld, do another new W-4 and add the extra withholding. (without knowing the formulas and your exact situation, there's no way to know how much things will change just from the new W-4, and if that might satisfy your needs without extra.)
Also and this is important, if you are married and your spouse works or you have more than one job, you need to submit new W-4s to all the jobs at the same time. Do one calculation, and then submit all new forms based on that calculation.
