The main objective of withholding is to ensure you pay enough taxes during the year to cover most, if not all, of your tax bill by December 31. Here is some useful information from the IRS that you can use when completing the new W-4 - FAQs on the 2020 Form W-4.
The amount withheld from your pay is determined by two things: how much you make and the information you provide your employer on Form W-4. Here is another helpful article about how it works. Form W-4 and Your Take-Home Pay.
Here is a link to the TurboTax W-4 Withholding Calculator to help you get your allowances correct on your W-4. W-4 Calculator
[Edited 01/16/20 | 1:11 PM]
In March 2020 I will begin receiving sick pay from my employer, paid through a third party insurer. The forms for the sick pay ask for a specific dollar amount to be withheld from the twice-monthly checks - there is no way to say "withhold at the old single-zero rate' or "withhold highest rate" etc. - they need an exact dollar amount. I know how much pay I will receive (without anything withheld) - how do I figure out what specific dollar amount they should withhold from each check so that I do not end up owing additional tax in 2021? I want it to be about the same as what used to be the 'single zero' rate. Thanks!
Here's an example of how that might work. Assume 20 paychecks of $1000 each. The Federal withholding tables show and amount of wage and the amount that shoud be withheld for the number of exemptions claimed. Claiming zero, the amount would be $1015 and so on. This particular line is for marrried weekly. You can see the tables yourself and find the figure that works best for your circumstance 2020 withholding tables
1,000 $1,015 $86 $76 $66