I don't understand why people find the pro rata holidays so difficult. I used to work out annual leave entitlements in my previous employment and it was very straightforward.
All you had to do was take the total number of annual leave days + bank holidays - in my case 25 days + 8 bank holidays, convert it into hours - again in my case 33 days x7.5 hours = 247.50 hours. (based on a full time 37.5 hour week)
Most of the staff were part timers so they were given pro rata number of hours holiday eg 25 hours a week = 25/37.5 x247.50 = 165 hours per year.
Out of those hours I would then deduct the hours of any bank holidays that fell on their normal working days at the beginning of the year and they then knew how many hours annual leave they had left to 'choose'.
So, for example, if they worked 5 hours every day, Monday to Friday, I would deduct 40 hours from the 165 hours giving them 125 hours to 'choose'. If they worked Tuesday 8.5 hours, Wednesday 8.5 hours and Friday 8 hours and Christmas/New Year was Thursday and Friday they would only be deducted the 16 hours due to be worked on Boxing Day and Good Friday because the other Bank Holidays fell on days they did not work, giving them 147 hours to 'choose'.
This also worked well for those who worked unequal days - eg 8 hours on Monday, 4 hours on Tuesday and 6 hours on Thursday - it meant they took annual leave based on the number of hours they actually worked - this stopped some people from always having their 'odd' annual leave days on the day they worked the most hours.
With this system there was no advantage in working certain days of the week to gain extra paid leave - the main problem was if someone worked the majority of their hours on a Monday - in this case we would allow them to build up some TOIL so they could at least pick some of their leave days.