I would always recommend Chappie for any dog with a sensitive tum, the vets always used to recommend it before all these dry foods were produced.
also our Chessie who nearly died because of tummy problems, survived because it was the only food he could tolerate.
If you are mixing it with JWB I don't think you need to worry too much about content.
My Goldie gets an upset tum if given too much Nature diet but is ok having it as half his diet