Audiothings - your simulations match what I get from VituixCad. I have an error in my excel spreadsheet ( 😡 Dammit!).
I would agree that the THX 85 dB average with a +20dB margin is a reasonable rule of thumb for a high fidelity system. Not sure about the +30 dB below <120 dB though. I would tend to express it more that the total woofer/subwoofer system needs additional margin for equalisation and sound absorption. The size of the margin is going to vary a lot with the type of woofer/subwoofer system. For example, no subwoofers and no equalisation would tend to mean a negative margin given the gain from the boundaries and volume plus room resonances. Whereas a distributed subwoofer system setup to maximise room mode absorption is likely to need a larger total margin than 10 dB from around the lowest frequency mode upwards.
Do you mean 85dB sensitivity across the audible spectrum? With what tolerance? -6dB at 40Hz ok? And with +20 gain do you mean peak or perceived loudness? These decisions will have dramatic impact on design. +20dB perceived loudness with music containing 10dB dynamics but you in a position where the whole system needs to be able to handle 1000 watt peaks on any frequency. I other words a large PA system. But then again you will have a much higher sensitivity and the whole calculation change again. See my posts on page 1 regarding dynamics, sensitivity and power handling might be helpful.
I was referring to the THX spec of 85 dB average SPL at the listening location with a 20 dB margin to handle transients mentioned by GM. This is a standard for cinemas and given the indifference of the music/audio industry to matters of sound quality is a common thing to look to for sizing speakers in the way OP initially seemed to want to do.
It is a minimum spec to meet for a loudspeaker system and so tolerances are not really applicable. However, at the frequency extremes near 20 Hz and 20 kHz it may be relaxed to some extent with little practical harm subject to some qualifications.
In terms of sensitivity low 90s seems to be around the sweet spot in terms of meeting the SPL requirement for woofer, midrange and tweeter without excessive power requirements or overly resonant cones. A bit less for subwoofers if sufficient numbers and cone area.
IME based on numerous and varied live events, cinema palace systems, plenty sure 😉, but assumed I would get some variation of your response 😀, so choosing from the various THX options based on one's intended app needs to be included, but my links are dead plus found out after posting that they've been revamped over time, so a belated follow-up: THX CERTIFIED LOUDSPEAKERS
Haven't had time to research further, but confidant that one, or even an amalgamation of some/all of them, will help folks choose what's ideally required to meet the needs of their app