I have no insight on levels for floating the J, but I do want to caution you depending on section. Story time:
When I went to PSU, I often fished the J. A buddy of mine and I were fishing in the Gorge section in water that was probably right at 500-600 cfs if I had to guess. In the upper section or the gorge, there is an area I like to call the minefield because it has tons of protruding rock with swift current moving between. We were at the lower part of this section when a guide and a client came through. Their raft hung up on one of the many rocks peaks which they must have not seen and the client went overboard. The client was standing and actively casting when this happened Was definitely scary as I was worried he hit his head. One rod and a couple of other items in the stream and the only thing they were able to grab was the fly line coming from the reel, it was a sh*t show.
Don't stand and cast in highly rocky sections. If I was fortunate enough to have a raft and float, I feel like I would definitely stop and wade a lot of the sections on the J depending on flow. But what do I know, I am a stinky wade fisherman.