I do agree with Sentoguy's remark that the rating system is not optimal. What I mean is that it's misleading - even fundamentally wrong - to base a trainees progress on the notion of "exercise density" in the terms that you put forth.
Really, 'Total Work Done' is a more meaningful way to quantify a total amount of reps * load. Attempting to chunk exercises into groups (Heavy, Medium, Light) in the manner you are doing isn't very accurate. It could be improved if you had a few scientists and an Electromyograph was on hand next time you were at the gym. The fact is, though, that exercises are pretty specific for strength (and hypertrophy) gains. The concepts of transmutation and transformation are applied in periodization mesocycles for just this reason.
This is wholly dependent on the goals that one has. There are so many directions ones weight training can go: Are you training for strength, speed, or hypertrophy?
Each direction requires focusing on different aspects of muscular development: strength is all about maximum MU activation and rate coding without regard for rate of force development, speed is about maximizing rate of force development in addition to MU activation and rate coding, and hypertrophy is all about increasing the density of myofibrils in muscle tissue and stimulating protein breakdown (and synthesis).
You seem like an intelligent poster, you are obviously thinking deeply about matters this site deals with. The truth is, though, that a lot of the information that authors deal with is regurgitated ad nauseum - and the rationale of authors' posts are never really developed completely because they are trying to sell you their products.
So! I reccomend that you read the Science and Practice of Strength Training by Zatsiorsky immediately. It is definitely a good starting point for anyone with even half a brain.