How do you calculate holiday entitlements for employees working continuous shift patterns?

We have people working a variety of shift patterns with holiday entitlements being historical due to various agreements over the years. However if we were to introduce a new continuous shift pattern what is the fairest way to working out what holiday they are entitled to.

For a standard employee who is days based and works 37.5 hours a week (9 till 5:30) they are entitled to 25 days holiday plus 8 bank holidays.
How would I work out holidays for an employee working a continuos 4 on, 4 off, rotating days and nights shift?

All help & advice appreciated.

Thanks in advance
Deenah Mistry