Supplemental method as separate payment Aggregate-No tax else percent

I have noticed over several years, If a termed employee gets a supplemental payment in the beginning of the year (so uses Aggregate-no tax else percent) method.  When it uses the W-4 it doesn't look at additional dollar amount.  It only uses the martial status and exemptions.  Does anyone know if that is for sure correct or should PeopleSoft also be looking at the additional dollar amount if it has to use W-4 for supplemental payment.   Any advice would be appreciated.