Extra Withholdings

My wife and I separated last year.  This year she is wanting to file married - separate which is resulting in me owing a hugh amount of federal taxes for last year.  I'm planning to adust my withholdings and pay extra withholdings each month.  When I did the W4 calculations online, the system generated a pdf that I can send in to my pension fund to adjust my withholdings.  On the generated form, the system automatically put $115 into the 4c cell based on my entered numbers (gross wages, federal tax withhold, etc).  Is the $115  an accurate number or do I need to change the number to what I calculate?