Well, there is no upper limit. That is determined by the player. But what you are being told is that for the most part flugel stuff will take place in the staff, where it sounds the most mellow. The mellow seems to reduce with the distance one gets above the staff. For me, a Curry fan, but others are saying the same thing about their faves, find an mpc company where you can be sure that the rim size and shape of your flugel piece is the same as for your trumpet. Tell the manufacturer which flugel you are playing and they will take care of the sleeve issues, usually. Then you are set to determine what you like in terms of cup depth for the flugel. Deeper equals more mellow and that may require more air than your trumpet does. You'll get used to it. Play the Wave chart down an octave. You can get it all except for the last E in the A section and it will sound much better. That chart (Aebersold? Real Book?) is scored more for tenor sax.