To keep it easy, let's assume a full-timer works at 40 hour week. You will have to adjust if the figure is different.
In a whole year, you are entitled to the same proportionate holiday as a full-timer. If you worked the same number of hours per day, it would be easy because you would just have 20 'days' holiday of your shorter working day. The problem arises when a part-timer works different days in a week or different shift lengths - that's when the calculation must be done in hours.
At the point you leave, you work out how much entitlement to holiday you are due. If the holiday year starts in September and a Full-timer leaves at the end of February, that's 5 months, so his holiday entitlement in that period is 20 days times 5/12. This is 8.33 days. 8.33 days is 66.64 hours holiday. But your working week is only 50% of a full-timer (20/40) so you get 66.64/2 hours. This is 33.32 hours holiday entitlement between Sept and February.
Then, every time you have had a shift in holiday, you deduct the length of that shift in hours from your entitlement - 33.3 hours. If you had a Monday off and that was an 8hr shift, 8 comes off the entitlement, if you had two Fridays off and they are normally 5 hr shifts each, that another 10 hours off the total.