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.
How U determine first and second reflection points
Someone told me following a while ago in room teak thread, but I don't think I understand it well. Any comments?
-------------------------------------------------------------
Have someone sit in the primary listening location, take a mirror to the side walls opposite each speaker and move it until the seated person can see the speaker reflected in the mirror. These are your first reflection points. Start from there.
-------------------------------------------------------------
-------------------------------------------------------------
Have someone sit in the primary listening location, take a mirror to the side walls opposite each speaker and move it until the seated person can see the speaker reflected in the mirror. These are your first reflection points. Start from there.
-------------------------------------------------------------