Hi Ayman,
1. First you have to create a evaluation group for each percentage rate in OAVA.
2. Define substitution for creating assets in OACS. Here you have to create a logic such as for each evaluation group default the depreciation key.
3. After creating a substitution, at the time of creating of asset master maintain the respective evaluation group and save it. Based on the group system will default the dep.key.
Hope you are clear now.
Regards,
Mukthar