We calculate the number of days holiday plus bank holidays due to a full time employee (20 plus 8 = 28 days in a year) and then calculate the number of days pro rata to the number of days actually worked. For some one working 3 days a week this would be 16.8 days. (28 / 5 x 3) This is then the number of days off they can have including Bank holidays. This is fair and seems to work.