What is the requirement for withholding on wages for the following?

Employee A receives supplemental wages identified separately from
regular wages.  Employer B uses the flat rate method of 25% for
withholding on supplemental wages.  If Employee A has a W-4 on file
with Employer B, are the additional wages specified on the W-4 taken
into consideration when calculating the 25% supplemental wage
withholding?

For example:

Employee A Bonus = $10,000.
Employer B withholds a flat rate of $2500

Or

Employer B withholds a flat rate of $2500 plus an additional amount of
$500 that Employee A has filed with Employer B on his W-4 form.

Which withholding method is correct and why?

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Clarification of Question by tigers-ga on 21 Apr 2005 12:34 PDT

After reviewing the last sentence of the 2nd section I realized that
it didn't read quite right.  It should read If Employee A has a W-4 on
file with Employer B, is the additional wwithholding specified on the
W-4 taken into consideration when calculating the 25% supplemental
wage withholding?