Tom, your results indicate that the levels you actually use will be no strain on your M2s, EP400, amplifier, and most importantly, your hearing.

An especially interesting result to me was the comparison of the levels at your listening position and at the back of the room. The studies of Dr. Toole and others which indicate about a 3dB lower level per doubling of distance in typical home listening rooms have been mentioned here several times. In your case the back measurement at 25' was not quite two distance doublings and about a 5dB lower level would be expected. Your results do indeed show that difference, varying from 4dB to 6dB. That app may in fact deliver reasonably accurate results.

Enjoy the music, not the equipment.