I am in a muddle. I have recently employed two people in a start up company. I have an accountant to do the monthly payroll but I need to do staff contracts and work out paid holiday entitlement etc. Both staff members work 4 days a week but the hours vary depending on the day of the week. One employee does 35 hours condensed over four days, with the hours varying according to the day of the week. Three long days, one short. The other will do three long days plus one shorter day on a floating basis, mutually agreed, depending on our needs. My head is spinning trying to work out how I calculate their paid leave entitlement and make sure it's all above board.
They will need to work some bank holidays.
Is there a formula you use that takes an aggregate across their working week to work out how many hours they should be paid for on a paid day off? Or do I pay someone for a 10 hour shift on a Friday if they take it off, and a 6 hour shift on a Sunday if they take that off? If I pay them to be off on Christmas Day for example, how is that calculated? And what if it falls on a day when they are not regularly scheduled to work anyway? 
Does anyone use an HR app or subscription service for this sort of thing? Can you recommend one and roughly what does it cost? The other option I guess is that I pay a one-off consultation fee to have this sorted for me, then manage it myself going forward, but i am not sure how I go about finding the right sort of service for this. What do other small business owners do please? Grateful for any input.