One thing to know that took me a little while to appreciate: The data in the Lee manual are a hodgepodge of data from multiple sources. I assume various powder and bullet companies gave Lee permission to re-publish their data. So, the velocities are not necessarily apples-to-apples comparisons. Some sources develop their data in a 10" pressure barrel, some in a 7" barrel, maybe even some in 4" barrel. There's no way of knowing which line in the table came from what length barrel, unless you go to the powder seller's website and look for the same data there and see what (if anything) they tell you about the barrel length.
So, while the fastest loads in a table will probably be pretty fast in a real gun, and the slowest pretty slow, you can't assume that the real-world velocities will line up in exactly the order you see them in the Lee manual, and your actual velocities will probably differ, usually lower.