Head of Household

Hello!  I am a head of household tax filer.  I work a full-time job normally but did a seasonal job the first 4 months of the year.  I do have an opportunity for bonus with my employer which is always taxed extremely high when it is paid out.  I am concerned with any increased income, the IRS worksheet tool is not going to calculate correctly what I need for withholding.  The multiple income worksheet no longer applies since I am only working one job.  Is it better to talk to a CPA about tax sheltering and withholding at this point versus just adjusting my withholding from the calculator online?