How is OpRA calculated when added mid year?

An employee forgoes a cash amount which is greater than their benefit (company car in this case), the cash amount is recorded in 'Amount Forgone' on the calculation card, which means they will qualify for Optional remuneration arrangements (OpRA). The change is effective from mid year.

I would expect the monthly BIK to be a calculation of Amount Forgone / number of months remaining, eg. 4400 / 8 = 550, but it's calculating a different rate, is that because it's now making the calculation on days rather than months?

If the annual amount forgone is added from the April payroll then that is what happens : 6600 / 12 = 550

Question : If added mid year, would it be calculation on a daily rate rather than monthly?