Either divide by the number of hours worked per week or by number of days. So if 2 day week then the salary is divided by 5 (5 days in a week) and multiply by 2 (days worked)
I hours then divide by 40 and multiply by 16 for 2 days per week.
sandy is right. Find out how many hours is full-time (say 37 hours per week) so if you work 18.5 hours, that equals half the salary of a full-time person.
I think that might depend where you work, anngel - at our place you get so many days BH included in your annual leave entitlement, with a "leave day" equalling how many hours you normally work per day.