I've never seen anything that's actually useful, and doubt whether I ever will, because light "losses" will vary depending on a number of different factors, and will be specific to different manufacturers.
Basically, "loss" of light (which in fact isn't loss at all, it's dispersion) is governed by the Inverse Square Law, which is a theoretical law which pre-dates photography and the artificial lights that we use for it. There are a number of conditions set out in the ISL that cannot apply to photographic lighting, for example it involves a point source of light, which we never use, and it occurs in free space, which is physics talk for a vacuum, where there is no pollution, and where there is no reflectivity from walls, ceiling etc. But, it works well enough to be practicably useful in photography except where very large light sources are used from a very close distance, e.g. a large softbox, where there are an untold number (in the millions at least) of individual point sources of light spread over a large area, which each point source of light travelling a different distance, and from a different direction, to the others.
So, the main factor that controls the amount of light that reaches the subject is always going to be most affected by the distance that the light has to travel. Everything else is secondary or tertiary.
What all modifiers do is to control the direction and perhaps to also diffuse the light.
A reflector controls direction, and also controls the spread of light, but different reflectors have greater or less efficiency than others. Leaving aside size and shape, the reflectors fitted to hotshoe flashguns are extremely efficient because they have an almost mirror-like surface compared to the reflectors fitted to studio flash heads, no doubt to produce higher guide numbers. Even with reflectors for studio flash heads, there is a vast range of different efficiency levels, for examples Bowens reflectors tend to be highly efficient compared to ours, which again produce a theoretically higher guide number. And our own high intensity reflector, even though it has a standard finish on it, produces between 2.5 - 3.5 times as much delivered light as our standard reflector, depending on distance.
Take beauty dishes as another example. Our white finish beauty dishes produce about 0.6 stop less delivered light than our silver finish ones, those from other manufacturers may be more or less efficient and the figures will be different because of this, and will vary even more because the beauty dish is likely to have a different shape, which means that the light will be spread over a different area than ours.
Softboxes - some have a recessed front diffuser that limits the spread of light, others don't. Some have thick diffusers, others are like tissue paper. Some have walls that are very efficient and shiny (but which don't mix the light up well) and others don't. Generally though, things like softboxes don't "lose" light so much as spread it over a large area, the light is "lost" in the sense that relatively little of it reaches the subject, the rest of it goes all around the subject and, with the cheap ones, some of it leaks out of the back and some of it leaks out of the walls.
Quite a few years ago I did do quite a lot of research on this, and I published the light "losses" from a range of our own modifiers at a given distance, but as we are constantly trying to improve our products things have changed, and even if that data was actually useful at the time, it won't be useful now. And it definately won't be useful to you.
