How U determine first and second reflection points

You have it correct with the mirror method, you can also use it to find the reflection point on the ceiling and floor. Go to audioasylum and click on the Rives site as there in lots of great info and recently a disscusion on this topic.

Sit in your listening chair. Have a friend take a flat mirror (e.g., the kind you use for shaving with or in the bathroom - hopefully at least 8-9" x 8-9") and place it flat on the right side wall just behind or beside your right speaker and at about the height of the tweeter in your speakers. While you are sitting in your listening position, have the friend stand behind the mirror and start moving the mirror towards you until you see the tweeter of the right speaker in the middle of the mirror. THAT is the FIRST reflection point. Then have that person keep going until you see the tweeter of the speaker nearest the left wall in the middle of the mirror (still on the right wall) and THAT is your SECOND reflection point. Do the same on the opposite wall and you have your first and second reflection points.

Does this help?

Between my post, Snofun3's post and Fmpnd's post, you have the same method nicely described by three different people. Pick one and have fun.

I found it easier to use a small mirror if I put a lamp (no shade) at the center of the speaker, then, as above, move the mirror until you "see the light".

Other choice is to sketch your room (to scale) and find the reflection point on the drawing (where the "angle in" equals the "angle out"--in other words, the angle the line from the center of the speaker to the wall makes is equal to the line from that same point to the listening position). Lamp is much easier...

I have not worried about 2nd reflections

So, each of the left and right walls will have 2 relection poinsts - one from right speaker and one from left speaker?