If the light inside is too dim for the meter to respond, then meter the external scene with the meter, and use as the aperture setting on the meter the lens's aperture diameter divided into the distance from lens to photo paper. Thus, you would be treating the camera obscura as a large view camera and would meter as you would with any other camera.
Depending on how you mounted the lens to a window or other external opening, you could build a flap-like door adjacent to the lens, which you open in order to meter the external scene's illumination.
PS: I would add that measuring directly the aperture of your lens, and dividing that figure into the focal length of the camera obscura, already takes into account the so-called bellows factor, so that no additional computation would be required beyond metering the external scene's illumination.
PPS: If employing the metering flap adjacent to the lens, you would meter the external scene through the flap with the meter set to its reflective metering mode - again, just like you would meter a scene when using a conventional camera.
Last edited by Joe VanCleave; 07-19-2012 at 12:12 PM. Click to view previous post history.