Electrostatically focused CRT G4 Voltage Range?

[Kevin Kuehn]

A typical G4 spec seems to be -50 to 300v. Is there an actual sweet spot in that range for a specific tube, or is G4 voltage so tolerant that anything in the ball park works pretty good? I recently tried varying the voltage between 0-300 on a 17BJP4, and to my eyes, I could not detect a focus change. I guess I don't understand why such a wide range of voltage is called out?