MACRS dep key, Half Year to Mid Year convention.

Hi Everyone!!

I am working on MACRS depreciation key and set up Half year period control method, But, mid-quarter convention is required when the total depreciable basis of MACRS property that was placed in service during the last three months of the tax year is more than 40% of the total depreciable basis of all MACRS property that was placed in service throughout the entire year. I am not able to figure out a way by which the period control method changes from Half year to mid year convention based on the rule ( 40% ), or is there some other way to manage this in SAP. 

Any input on this issue will be grately appreciated!!

Thanks