That's a pretty good explanation of how to find the first reflection point. You, as the listener, sit in the listening position. Have another person with a large mirror (3'x4' would be nice...or you can use reflective Mylar sold at Party stores), place the mirror on the side wall next to the right speaker. Have your helper slowly slide the mirror along the wall moving toward the listening position. As you look in the mirror, there will be a point at which you see the right and left speakers reflected in the mirror. Have your partner mark the wall with some blue painters tape at the spot where you see the reflections of the speakers. Your partner will put two pieces of tape on the wall...one for the reflection of the right speaker, and one for the reflection of the left speaker. You will have to "talk" your helper to the correct postion, because obviously only you can see the speaker's reflection. If you use reflective Mylar, you can tape the Mylar to the wall and put the blue tape right on top of the reflection. The blue tape now marks the point of first reflection of each speaker on the right wall. Repeat for the left wall.
For further info, read Robert Harley's book, The Complete Guide to High End Audio.
For further info, read Robert Harley's book, The Complete Guide to High End Audio.