How to prorate changes in accrual formula

Content

Hi 

We have requirement where based on work hours change it should prorate the accrual value , we are using Global Absence Accrual formula (not matrix) , so I don't think I can use event date formula here

It is semi monthly repeating period

Work hours -- 24 -- Accrual for 15 days -- 10 , work hours changed to 30 on 28th May , so it should consider 13 days as 24 and 2 days as 30 when calculating but in my case it is taking 30 for entire 15 days as 30 is current value as of 31 May 

Can you guys suggest a way to handle this

Thanks

Suresh