Pro Rate convention

Hi All,

We are on 11i. Currently we are using the mid-month convention and STL depreciation for all our assets. As of now full depreciation is taken from the first month the asset is placed in service till the last month of asset. We now need to make a change in system where the first and last month of asset would take half month depreciation and the full month depreciation for the rest of the months in between.

 How do we achieve this? Do we create a new calendar in the system to calculate half month depreciation for the first and last month of asset and then attach this calendar to the mid-month convention?  How will this affect the existing and new assets in the system? Is there any other solution to get the required result?