Option to choose a different Pro-rata rule

I am currently struggling to make sense of what pro-rata rule PHR uses for its calculations for holiday, especially when a working pattern is changed mid year. The knowledge Base article states it is based on n/365, however when you look at the calculations behind the '?' button, it states that it is the number of weeks (not days) in each pattern that forms the basis of the calculation.

As a result PHR continually gives more holiday than our contractual position of 1/260 and we have to manually over-ride it (and why have a dog and bark yourself?).

In other systems I have used, you can set the pro-rata rule you wish to use in the system settings and it works across the system in all pro-rata calculations.

Would this be possible to set up?