I'm not sure if I agree with using 3 dB gain from placement to estimate SPL. Wouldn't this only apply to lower frequencies, which is *below* the frequency where sensitivity is rated? In other words, will the speakers be + 3dB in room from 1KHz to 10 KHz for example? My understanding is that this is not the case.
Another factor that underestimates the amount of power required to achieve the desired SPL is that with good room eq, we can almost be assured of 3-6 dB of eq being applied below 3-500 Hz depending on where room correction is limited. So this doubles or quadruples power requirement. This is potentially up to a 9 dB difference with the estimate provided by the SPL calculator. It is my opinion that these are some of the reasons why high sensitivity speakers and/or lots of power are necessary for true, low distortion reference capability. Capability requirement is almost always vastly underestimated. It's so common to hear ZOMG my ears are bleeding at -15 MV it runs me out of the room. But I've been in a fully equipped JTR home theater where reference level was dynamic, powerful, and concussive, but was *not* uncomfortably loud. When I had 86-87 dB bookshelf speakers and a $200 AVR, -10 MV was painfully loud.
I do see your point, to a point..
I would still say that 3 dB room gain is reasonable, and optional, for a lot of home theaters where subwoofers are typically used. To error on the safe side, I typically (when do it for people who asked..) either use actual measured sensitivity figures or de-rate it by at least 2-3 dB, up to 6-9 dB for Klispch speakers (only those rated >92 dB/2.83V/1m) such as those RP-800F. And I always do it based on one speaker (THX standard). To be even more conservative, yes it can be ignored.
Regardless, room gain effects does affect the user, who would naturally adjust the volume control accordingly to get his/her desired spl bass on his/her speaker's in-room response, not anechoic
... After all, the job of the amp is to give the output the speakers need to produce the desired spl in a room. When you measured the spl when listening to music, or movies, you obviously would capture the spl created by the overall sound spectrum without filtering out the low to upper bass frequencies.
The 86-87 dB bookshelf speakers and a $200 AVR tells me such a combo may not do very well in maintaining a balanced frequency response for your spl need. For example, f you cranked the volume up loud enough, the speakers may end up producing mostly mids and highs that we are much more sensitive to then we are to the low to upper bass frequencies.
"Capability requirement is almost always vastly underestimated."
I am not sure what "capability" you are referring to. If it is "amplifier capability", then I wouldn't say it is "almost always.....". It is complicate, as it depends on many factors. I have seen it both ways, a lot of posters do tend to be quick making general statements, such as "most of the time people only use a few watts.....", to "double the power only get you 3 dB spl gain, amps clip much more often than people realize....". I know my amps don't typically output more than 0.25 W average, though I have seen maximum peaks to a little over 50 W when I listened to certain things and felt like listening to 75 dB average or higher from 11 ft. The fact is, if in one of my system I only need 0.2 W average, then I would need 20 W for 20 dB peaks on top, but for someone who needs 2W, then 20 dB peak would bring it to 200 W. So again, hard to say if it is in fact almost always underestimated, the opposite could be true too, I just don't know.
It depends so much on distance, spl desired and the speakers characteristics including FR, impedance, phase angle, sensitivity, maximum spl at 1 meter and maximum power handling capability etc. Your point about Room EQ is another major factor that complicates things even further. Fortunately people who use REQ for movies and music typically have subwoofers so in most cases, the effects of the boost would likely apply to the amps in the active subwoofers. Also, in general I would try my best to avoid boosts, when using Audyssey with the Editor App.