Is there any other way of prorating the timeline while invoicing a multi-period accounting invoice?

There are two seeded options available for prorating a timeline while performing a multi-period accounting invoice, which are mainly (i) Proration by Period and (ii) Proration by Days. Now, our business requirement is not being catered by either of those options. A practical scenario of the primary requirement of business is as being followed :

If timeline of an AP invoice is 4 months and 15 days now business wanted to consider the 4 months as equal 4 periods as per the proration by periods along with those 15 days as only 15 days not as a whole month period. If we follow method (i) from above then we can't calculate 15 days separately and if we follow method (ii) then business wants to ignore the extra 1 day comes up in period which contains 31 days i.e. they simply wanted to consider the 4 months as 4 equal periods.