The reflector above the light doesn't affect the measurements, since the light measured 4 " below the bulb and 8" below the bulb are both recording the reflected light, in effect the reflector just ups the starting values, its the reflective walls that make the difference to the measurements. If your light reflector aims the light at the width of the canopy, the walls will make little difference to the measurements, but if your reflector is high enough to allow some of the light to be hitting the walls before the plants, the measurements will change. To do this accurately though, the measurements need to be taken without walls and with walls for a comparrison. If someone with a light meter could measure half way from the bulb to the walls, then take readings at the half way point and then at the wall, (providing the light is hitting the wall at a higher point than the measurement is taken at), then that would be a good measurement to have.