I'm a noob and can barely get above the C in the staff... with any tone. But I have noticed that for me it is easier to hit those notes piano or even mezzopiano than full force. I don't really seem to need much air once I figured out it's less stretching and more scrunching of the lips for me.
I'm taking it real slow, just playing my scales and going for that next note. Being a bari player from way back when it is a different strategy. Once I figured that out, it's been much better.