Depending On Quota Arrangement PR to PO

Dear All,

PR is cretaed manually or automatically generated through MRP.

This this assigning of Source is done by ME57 and i want to pick on quantity percentage basis.

Like Ex - PR generated for 1000 Material and for this material source should to picked depending on percentage basis.

quota arragement and source list maintained for 1000 quatity as

400 - Vendor X

150 - Vendor Y

550 - Vendor Z

How the system will be picked the vendors for the the PO generation with the sources on % basis which is maintained in Quota arrangement.

Explain me the flow

Thanks