All boxes fall off at exactly the same rate. One of those pesky physics laws there is no way around.
Along the lines of "how many people will that speaker cover"
In the case of the SM80-it will cover 1 person pretty well (if facing straight down). 2 people if they are close friends.
It is not the number of people-but rather the spacing of the people-AND how loud the program content is if the real question is "How many people could be in attendance and still hear good".
It is one thing to be doing jazz and another doing rock and roll. And are the people seated at round tables or standing close together?
The same speaker will give VERY different results.
Hence the need to PROPERLY DEFINE the needs. Loudness-distance-width etc. Distance and width are pretty easy-but getting a usable target SPL number is quite another. Is that number continuous-peak-what weighting? Is it realistic? Are we allowing "extra headroom" for the loudspeaker so it doesn't blow up?
What about the "sound quality SPL"? Just because a particular speaker can produce 130dB, does not mean that you can listen to-or would like the sound quality of it that loud. It might be that 125dB (or less) may be as loud as you would want to run it. So that number (that you won't find on any spec sheet) is the number for the calculation. And different people have a different idea of what is "usable".
Of course that is NOT the answer people want to hear. They typically want a simple-easy to understand WRONG answer.
Sorry-but that is just the way it is.