As answered on GS, there’s no (real) loudness spec for audio in a movie theatre.
But to get you in the ballpark, and to quote Georgia: “Get the dialogue to around -12 for yelling with a bit higher for the occasional scream, and down to -22ish -20 for speaking, down to -23 (and lower) for whispers.
Big crashes / hits/ explosions can tap 0. Quiet ambience can get all the way down to -28 ish or lower”
But again, these numbers are not a spec. They’re based on personal taste, with a certain type of film in mind, and should be measured ‘True-peak’.
Also know that theatres have a so called ‘X Curve’ on their sound-system, meaning both low & high frequencies are rolled off to a certain degree, but not equally on every level. When mixing you can compensate for that X-curve.
Another thing to keep in mind: when mixing over nearfield monitors, it’s difficult to create a dynamic sounding mix, and quite impossible to mix low frequencies.
It’s best to do a test run in a local theatre (or calibrated dub stage) before signing off on a project.