If she works part-time then her holiday allowance should be in hours not days.
You need to take the number of hours a day full-timers work (i.e. weekly hours � 5) and multiply by 18 to get the number of hours leave a full-timer gets per year. Your part-timer should get the pro-rated amount depending on the proportion of a full-time week she works.
So if a full-time week is 36 hours that makes 7.2 hours a day for a FT x 18 = 129.6 hours. PT does 18�36 = 0.5 of a FT week so gets 129.6 x 0.5 = 64.8 hours leave per year.
Of course, if she works the same hours each day, it's much simpler - just multiply the daily hours by 18 to get the total.