Would it not be easier (and cheaper) to make a diaphram? A fixed one anyway. The F number should be straight forward math, I believe it's the ratio between the focal length and the diameter of the aperture of the lens. (http://www.uscoles.com/fstop.htm) You are making a camera, you can drill a hole. This will insure detail on the image, predictable results on the film and a whole lot less screwing around.

Just a thought........