There are just extension tubes, right? None of them are a teleconverter, i.e. there are no optics inside?

If that is the case, they do nothing except add to your existing lens' focal length. That's it. Light loss incurred is only due to the increase in focal length because your aperture obviously still has the same maximum diameter.

The only Fun aspect of this that I am aware of is that the ƒ number markings on your lens will now be wrong, because they were calculated by the manufacturer using the lens' original focal length.

For those of us whose minds are regularly blown by this math, including mine, ƒ-stop numbers are literally the fraction of your lens' focal length divided by the iris diameter. The markings do not tell you the actual iris diameter, which to be fair would not be actually functionally useful information when you're standing there trying to take a picture in the field, but it's a number you then need to back figure when you start doing stuff like this. The entire point of this kerfuffle is that any given setting, say ƒ/1.7, should allow the same amount of light to hit your sensor/film regardless of what the focal length of the lens you have attached actually is. But that's only true until you start mucking around with changing the focal length with extension tubes and the like. Digression concludes.

You can calculate this by first determining the magnification factor of the tubes you've installed, which is the extension length divided by your original focal length. Let's call that m.

You can then calculate the effective aperture by:

ƒ x (1 + m)

Determining the actual light loss requires logarithms. You probably want to make sure your calculator has enough buttons for this one:

2 x log2(1 + m)

...Or you can just compare the new effective ƒ number versus the old one. Or use your camera's light meter if it has one. Or just declare the heck with it, and use any of the available calculators for this sort of thing.

Or wing it. Winging it is what I usually do.