I've also been making density-curves lately. See the thread about Progress on XTOL-Concentrate. I've made and corrected some mistakes.
I agree with Michael R: The long exposures are likely to cause reciprocity failure. I suggest that you photograph a brighter light-source. I'm using my light-table: It's bright enough to keep speeds at/above 1/4 second, and dim enough to capture the film's toe. Also, consider putting some neutral density (ND) filters on the light table to give you multiple densities with each exposure. A step-wedge would be ideal, but ND's would help if you have some lying around.
And as you realize, avoid the corners of the neg due to fall-off due to the lens and also due to the lens' viewing the light-table at an angle.