Form DE 4 has a couple worksheets on it to help you figure out the amounts you should enter for withholding allowances, but it doesn't give you any way to see how those will actually affect your withholding. It just says "After your Form DE 4 takes effect, compare the state income tax withheld with your estimated total annual tax." So to see if you are withholding the right amount, you have to wait and see what happens with your next paycheck.
That's pretty slow and inefficient, since you might have to wait weeks or months to figure out if you need to adjust your withholding again. And, of course the State does publish the withholding calculation; it's just a tedious PDF.
This calculator implements the "Method B - Exact Calculation Method" in that document, so you can see how adjusting your DE 4 will change your withholding without having to wait for your next paycheck.
NOTE: Use this calculator at your own risk. Some validation has been done against real paychecks, but there could be errors, and parts of the document from the State are ambiguous. So you should still verify that your withholding ends up as you expect it to. Also this should go without saying, but changing your withholding does not change your actual tax due. If you set your withholding too low, you'll just have to pay the difference at the end of the year, possibly with interest and penalties.
If you have any questions or feedback, feel free to email.