One-way TYO-PVG-HKG in Y should be 20000 miles in my understanding. If they count TYO-PVG and PVG-HKG as two separate trips (which I think would not make sense), it would be 27500 miles.
Note, however, that one-way awards can only be booked for flights operated by Air France, KLM, Air Europa, Kenya Airways and Tarom because one-way awards are offered only by the airlines that have Flying Blue as their FFP (except Aircalin, which, not being a Skyteam member, is not fully integrated). (A return flight in Y would obviously be 40000 miles or 55000 miles.)