While I'm sure the manual has things like "this lens with this extension can focus this close." but you don't really need that. Pick a lens, mount it direct to the camera and focus as close as you can so the lens is racked as far out as possible. Then watch the viewfinder as you walk closer to an object. When the object is in focus, that is your minimum distance with no extension. If you're shooting anything with that lens that is further than that distance don't use an extension--lets say 30 inches or greater. Next attach the shortest extension to the lens and still with focus racked all the way out, do the same thing--lets say it lets you to 15 inches. So use that extension for anything from 15-30 inches. Do that for all your extensions and you should be able to come up with a simple table: infinity to 30": no ext; 30-15" ext-1; 15-8" ext-2; etc. If you lke you can also eyeball the object size if you want to know the magnification as well.
The thing to remember is extension tubes affect close focus distance, that it. Because the size of the object in the viewfinder is directly affected by how close you are, reducing the minimum focus distance effectively allows you make object larger. Since they are just empty tubes, they don't change the optical properties of the lens, they just allow it to focus closer than you normally would.
One other thing to remember is if you are using an external meter to get exposure, you may need to calculate the so-called "bellows factor", thanks to the inverse square law, the further you move the lens from the film, the less light falls on the film. When you get to 1:1 you need to increase exposure by 2 stops. If you are using a TTL meter, then you don't have to do that because the internal meter already sees the reduction in light and will give the correct exposure.
