I would stick to the late Col. Cooper's FOUR RULES mentioned above.
They are sufficient, with the understanding of range procedure;
That when the RED flag is up, the line is hot, and firing may be in progress.
When the WHITE flag is up, the line is cold; there is no firing, all firearms on the firing line (i.e. laying on benches etc) are clear - unloaded, people are downrange.
You could add a LOUD buzzer or horn to announce changes to line flags, or just have the assigned range supervisor to shout a "Cease fire cease fire!!!!! The line is cold!!! No firing etc!" And a visa versa for the red flag order.
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Je Suis Prest