... could you tell me where a can find the good practice and parameters for create a VM with Convertor ?
You'll find most answers here in the forums 
Anyway, I don't know of a best practice. There are a few things however to keep in mind, where the most important part is the virtual disk controller.
- Windows 2003 -> LSI Logic Parallel
- Windows XP -> Buslogic
- Windows 7/2008 -> LSI Logic SAS
If a conversion fails at 98%, check/fix the VM's configuration and run the Converter again, using "Configure Machine". This often helps fixing boot/controller issues.
André