exempt but add an amount for witholding

Is it legal to claim exempt, but set an amount for withholding? Rather than claiming married 2, or single 0, etc.,  can you claim exempt and set an amount to have taken out of your check under section 4c Extra Withholding on federal forms.  For example, Exempt, and 500 in section 4c for the amount to be taken out. technically you are still reporting and paying taxes that way, just not sure if it is legal to do.