Jim, don't feel like you are the only one to wonder about all the "rules of thumb," science, and advice unsupported by evidence that we receive as anglers. It is a lot to sort through at the beginning. Eventually, due to your love of the game you will sort it out and find what works for you. As some have stated above, but in other words, there are the following:
Real science.
Rules of thumb that may apply more often than not due to that science.
And also pure fishing conjecture based on that science.
Avoid number 3, and don't forget that number 2 only applies some of the time. Take the real science, apply it on the water, and as you spend time on the water you will work out some things that work for you. I know that is a very general statement but it is why so many mention "time on the water" because there is an art to learning to apply the science so that it becomes second nature . . . . almost instinctual, and that art is developed only by time on the water.