PT worker holiday calculation based on days not the hours

at the moment PT holiday calculations takes into account hours worked per week. but if i have a PT employee who works 5 days a week 20 hours as opposed to 40 hours for FT employee, the PT employee will still be entitled to the same amount of holiday days as the FT employee but at lower daily rate. this is currently something that is calculated incorrectly as employee who works 20 hours over 5 days will be by default given half of the FT entitlement.