Your profile says 2010, so I would:

1) Compute the desired percentage somewhere adjacent to the values in the spreadsheet.
2) Use Rob Bovey's free XY chart labeler add-in (appspro.com ) to add these cells as data labels to the stacked bar series.

If, perchance, your profile has not been updated and you really have 2013 or later, then I would do the same thing, just use the built in "value from cells" feature in the format data labels dialog.