Paula works 25hrs per week part time. Full time employees work 37.5 hrs per week with 20 days holiday per year, so I believe the following calculation should be applied to work out her holiday entitlement:
25 / 37.5 x 20 = 13.5 days holiday entitlement (rounded up).
Question: is that 13.5 days of Paula days or full time days (8 Hours per day)?
No it would be at equivalent of full time days, hence the pro rata.
Essentially she should have 20 days as anyone else would, but at reduced hours. so 20 x her hours per day should equal 13.5 (or your original figure before rounding) x 8 hours.
My leave is worked out in a similar way, and I'm given figures in days and also in hours. So when I book leave, I have to calculate it in hours as well as days. The days element is really only there so that my manager knows what days not to expect me in.