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.
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.