As trainers, we can take the time to digest all this info and evaluate each situation as it arises. As far as the average forklift operator is concerned, he/she will not take the time to establish the actual weight of the load vs. the capabilities of the forklift. Although they should, in real life, you, I, and a host of others know better. They will pick up any load and determine whether the back end starts to lift.
The 1"=100lbs is a simple, straight forward determination that most can understand so the operators can get a general idea as to whether they are within the lifting capacity of the forklift.
It is best to keep it as simple as possible because if the info goes over the heads of the operators, they will not listen, and worse, not practice what is taught.

And this contiues on to many other topics on this board where the letter of the law is quoted, and this is the way it is. If you understand where many of these operators are coming from, then you will soon learn to realize that they do not want to be in that classroom. They are there only because they HAVE to be there. And if an instructor is going to complicate the issue, the attendees will snooze. They want in/out in the shortest period of time (my students have to be patient) but if the topic is kept light and easy to understand, you may very well win them over. But you cannot complicate matters. We are not dealing with engineers, doctors or scientists. Most of my students have not attended college, and some did not even finish high school. And being too specific on many matters pertaining to forklifts, will definitely confuse them, resulting in the students ignoring what is being taught.

Yea, maybe manufacturers should place some type of chart explaining the capacities based on a variety of circumstances but I am sure that many other safety instructors on this board will agree that many of their students are not even aware of the lifting capacities of the trucks they are operating. Sad, but true. Do you think they even bother to read all the warning decals on the truck?

So, what is required is an easy formula for them to quickly determine the weight of the load they are lifting based on load centers greater than 24", and in most cases, the 1"=100 lbs is the simplest and easiest to remember. It may not always be accurate, but it does fall in close proximity to keep these operators from overloading the forklift, killing themselves, or someone else.