Also, there's a quick and dirty method of calculating twilight factor, which is what most manufacturers will use when they list it in their product specs.

Twilight factor = Square root (Objetive Size x Magnification)

Without getting into too much about lens coatings and glass quality, I will say that the calculation is usually pretty accurate. Two scopes at 3x with 40mm objectives will have a twilight factor of 10.95, without accounting for the type of lens coatings used, number of coatings used, whether those lenses are fully multi-coated and number of lens elements. Most manufacturers at this price point will claim between 97% and 99.5% light transmission, so at those levels it's very subjective.

That baseline calculation is used commonly in astronomy, and those guys have so little light to gather in the first place that the comparison between photographing a distant galaxy and scoping elk through a rifle scope at 7PM in the fall isn't even fair.

Regardless, I'm still going to try to get some real-world numbers because I think it would be very interesting to see how brands stack up against one another.

Edit: I realized later that what I said about twilight factor made it sound like it was a real-world measurement of light transmission, and that's not the case. It's just a measure of theoretical effectiveness in low-light.