Calculating pro-rata bank holiday entitlements for part-time staff

What's the best way, Seamus, to calculate pro-rata bank holiday entitlements for part-time staff? Is there any recommendation in law or good practice?  Lots of methods are out there on the web. For example, one of our staff has just reduced their hours from 35 hours to 30 per week from 5 to 4 days. So she is working more hours per day than the other staff, but over a four day period. I know how to calculate it. But should the calculation be pro-rata on days or hours worked?

Seamus:  Well, this is always a relevant question because we get so many queries about it. And Rolanda was saying, just when we were talking previously there, that when she was in the Labour Relations Agency,