You might want to get into the habit of weighing each batch. As bean densities vary greatly, volume just isnt a good way to measure beans for your roasts.
Pick up a small kitchen scale, digital if you like, I use analogue out in the garage. That way you can at least maintain some sort of consistency between roasts (and that is really important).
By the way, you might also want to make a "catcher" for chaff/renegade beans. A good example is my "popperdom." I would recommend using metal flyscreen, as you can cut it to the shape/size you require. You just make a sock-like shape, closed at one end, bent at 90 degrees to create a reservoir to hold chaff. Simply slip it over your chimney, and off you go.
Some pictures are here: http://coffeesnobs.com.au/YaBB.pl?num=1126057714