Your math problem is giving me tired head. The meter will read that correctly, so you can put your fears to rest. Here is something the meter will not read and which will have an effect on the situation. A principle of lighting is that the angle of incidence equals the angle of reflection. A light from behind skims off the subject and into the lens. It will always appear brighter than a light from the front of the same intensity. In fact, a hair light a full stop down from the main will still appear brighter from camera angle. The meter (assuming incident) does not read this correctly as it is always pointed at the light. A spot reading from camera position would read this light correctly. A very sensitive and exact meter is the human eye. It is a great comparitor. You can easily judge the relationship of the lights visually IF your model lights are proportional to the flash. The meter reads quantity of light, which the eye is not so good at. I meter the main and set everything else visually. Special cases would be the only
exceptions. In my image, all lights are the same intensity.