.............When you remove the front cell of your lens, you essentially throw the nodal point some distance in front of the front standard, and you need extra bellows extension to get to infinity. For your 305mm converted lens, it might be 350mm--maybe 400mm--depends on the lens. In use, it's like you have the reverse of a telephoto design: you need more bellows extension to use it rather than less.
That clears things up nicely. I can only assume that I can't use the lens as a 305 on this camera.