Looks like everybody is at CES, so it is pretty lonely here.

That chart is totally inaccurate, since it is just a general guideline. I have a sub inline with my main speaker, and from the speaker size setting, I should set my speaker to 11 (12" vented sub shared by 2 speakers, so it would be 2 x 12" + port = (20 + 2)/2 = 11 ?) Why would I want to set my mains to speaker size 11 when the sub itself could produce over 115dB?

Just follow what Meridian said, test with freq sweep build into the processor and set it to one size below where the driver bottoms out.

If you got all the time in the world, you could do a freq sweep with different speaker size/dB settings. You don't have to use your ratshack meter and risk blowing up your speaker/amp. Just take the voltage reading out of the preamp or power amp w/o any speaker attached.