Do not claim exempt unless you are truly legally exempt for some reason.
The W-4 is meant to be self-adjusting. If you get paid more, more is automatically withheld and if you get paid less, less is automatically withheld. If you are finding you have too large a refund (too much withheld) or owe too much (too little withheld) it is best to follow the instructions using all the worksheets on all 4 pages to figure out what you should claim. You can also use the IRS web site calculator to enter your year to date tax info and the web site will recommend how to file a W-2 for the rest of the year.
https://www.irs.gov/individuals/tax-withholding-estimator
