Multiply the employee's wages by the desired percentage and divide by the number of pay periods per year.
Then use the paycheck withholding calculator to figure out the number of exemptions that would result in the desired per-paycheck withholding amount. And remember that, for example, if $60 needed to be withheld per paycheck but X exemptions only provided $50, you could use X exemptions with an additional $10 per-paycheck withholding (per the "additional withholding" on the W-4 withholding certificate).
At least, that's how I would do it. Shucks, it's probably how I will figure my withholding amounts when I start working in late June.
I AGREE WITH RABSCUTTLE, who said it much more clearly than I might have been able to.
Thanks!
I cut-and-pasted your post and told Jorge to deal with it.