Do not judge only by the highest note
Some songs do not go very high, but low notes or long phrases can still make them unstable. A good key fits both range edges and breath demand.
Control matters more than touching the note
Touching a note once does not mean the song fits. A controllable note is repeatable, not squeezed, and does not collapse at the end.
Choose a practice key before a performance key
For practice, lower the key if needed. Stabilize the hard phrases first, then move closer to the original or performance key.