Jim,
You really are making things complicated for yourself. Why not just use the equation for density (from any aerodynamics text) ? Also, the 118.8 ft/C° dev factor is very rough and ready although we routinely use it (actually rounded off to 120) for DH calculations in pilot speak. You may find the following thread on Bob Tait's forum of some use to your deliberations
Bob Tait's Aviation Theory School - Altimetry Rates - Bob Tait's Aviation Theory School Forums
So far as posting stuff like graphs, photos, etc., there is a limit which requires you to have a number of posts before the site lets you. I can't remember the figure but it's probably something like 10 posts. In the meantime, if you email stuff to me as attachments, I can put them into your posts in the interim.