A blackout is the only cue that has to be exact. Everything else has tolerance; this one does not, because the audience can tell instantly whether the room is dark or nearly dark, and nearly dark reads as a fault.
In practice, getting a public building genuinely dark is mostly a negotiation. Exit signage is a legal requirement and is not going anywhere. Practical work then becomes about controlling everything that is negotiable: the desk’s own screen, the followspot’s tally, the glow off a music stand, the light spilling from the wings when someone opens the pass door.
Not how dark, but how long. Under about four seconds a blackout is punctuation. Past roughly thirty it stops being a lighting state and becomes an event in its own right, and the audience will start to shift and cough. Between those two figures is a real interval with real dramatic weight, and it is worth spending time in rehearsal finding out where inside it the moment sits.
That is genuinely enough to start a conversation. Dates and budget help, but they are not the first question — the first question is what the room is being asked to do.