This is the most accurate withholding calculator if you can come up with an accurate estimate of your income and deductions for the rest of the year.
https://www.irs.gov/individuals/irs-withholding-calculator
Remember to run it again in January because if you set your allowances high enough to reduce your refund for the rest of 2017, you may not have enough withheld for 2018.
A rough estimate is that you should add one more allowance to your W-4 for every $1000 of your tax refund that you would rather have in your paychecks.
If you actually take the time to complete the worksheet on the form W-4 it's usually fairly accurate as well unless you have special circumstances.
