Some years ago I needed a good way to support steel bars that were to be welded, collared, etc.. to make gates and railings. Things needed to be really flat for several reasons, which, I'm sure are obvious to most of you. I used some saw horses that I have had in my shop for years, but it was hard to keep them from moving since the floor was not perfectly flat. Also, as I moved around the area with my helmet on, I'd occassionally kick a leg and knock the whole thing out of alignment. To get things perfectly level, I had to constantly be shimming the components, by as little as 25 thou here and there to make up for all the variables. I chose to make some adjustable 3-Legged saw horses which solved the uneven floor problem and eliminated the need for shims. By uneven, I mean uneven by only a sixteenth of an inch or so in 4 feet. My mentor Bob Walsh, from Pepin Wisconsin also showed me how to make and use "winding sticks" to make certain that both beams of the saw horses were parallel (in the same plane).
Here's where you can learn more about it: