R12 Fixed Asset - Spécific depreciation methode

My customer would like to create specific depreciation method and be able to update actual assets not yet depreciate by this new method.

Specific depreciation rules expexted :
1. The depreciation should be calculated based on the NBV instead of the actual cost of one asset
2. The NBV shoud be at the end of the fiscal year and not at the transaction date.
3. The depreciation methode is  based on STL method expressed in Years and month depreciation time, like the old one.

How can I create easily a new depreciation method based on those criteria ?

Best regards