First reflection formula?

I've been using the "mirror" on the wall system, I though there was a formula for where the first reflection is based on the speaker distance from the back wall or something like that. Anyone know what I'm talking about, or has the smell of fresh vinyl gone to my head again.
Formulae are useful for all sorts of things. For example, they can tell you at what frequencies standing waves will occur. As far as determining where the first reflection is located, I don't know if there is a formula. However, it's not necessary. Use your "mirror on the wall" technique. If your ears and speaker can "see" each other, it's a reflection point. Measure the distance from the speaker to the mirror and from the mirror to your ears. Add the two figures together. No need for a formula; it's just arithmetic. Whichever reflection has the shortest total distance from speaker to ear is the closest reflection.

By the way, the mirror should be placed on the ceiling and floor as well as the wall. The first reflection is likely to be from the floor in front of the speaker.
Yeah, it's the fresh vinyl.

Use the mirror method if you have someone to move the mirror while you sit in the listening position. It's the easiest way. Oh yeah, and it's OK to look directly at the mirror, you don't have to keep your head pointed forward ;-)

If you're alone :-(( here's a good approximation: measure the distance from your ears when seated, straight forward to a point between the speakers (you can do this on the floor since you're ALONE) Then, mark a point on each sidewall which is exactly opposite the midpoint of this line. This is easy to do, even if you're ALONE. First, mark the midpoint of the line on the floor with a piece of tape. Then take the tape measure and measure the distance from the back wall (behind the speakers) to the midpoint mark. Write it down. Then, measure off that same distance along each side wall (from the back wall) and mark the spot(s).

If you start your absorbtive material/panel/etc. at this point on the wall and extend it toward the back wall by 1.5 feet for every 5 feet of (total) room width, you'll be right on.

I asked the same question over on AA a little while back. I had seen the formula somewhere, then when I needed it I couldn't remember what site I saw it on:

It's down near the bottom of the page.

If you really want to try something interesting, read this review of Eighth Nerve products, which also discusses Nathan Loyer's speaker positioning method. While I just started using his products, I also use absorption panels (not on first reflection points) and after following his method, the improvement in clarity was quite noticeable.
ever try the tin foil and a light bulb on your wall?, the foil will allow you to see reflection points very well.