Your understanding is correct, but the percentage question is a little confusing. I think the writer is trying to ask about the overlap of these two concepts, and asking you to express this overlap in a percentage...which seems almost impossible to do!
By the way, I can think of "prescriptive" as related to a "prescription" that is written by a doctor, telling you to take a medicine.Would you like to discuss this more? If so, I will think about it and answer again! :)