ok guys,
I finally found the way to derive this BP rating:
I added standard characteristics GRADE and GRADE_METHOD to my analysis structure.
I created a new characteristic WYQ20S (i.e.) based on original field BP1012-DATE_TO in order to complete my BP1012 in derivation rule source fields (the 2 other fields are useless so with initial value allowed).
Then it is rather simple to create steps for deriving this new limit characteristic.
Thanks
Rgds
Philippe.