Dan, the method you describe is pretty much what I was taught. Level your horses, then put your timber on them. Then put a framing square on each end, and use a penny or a nickel to adjust each of them so that the tops are in the same plane. Then mark your 2" mark, and use a combination square to mark 2" down on the side, and on the bottom. The bottom mark is tricky cause your square wants to swing and move. Do that on each end, and snap lines between the marks. I guess, the way others are doing it involves levels and marking the ends, but that's not the way I learned. What you have described for the live edge beam is right on -- basically you're choosing where the level line is on each end, then snapping a line, and laying out everything from the level live you've picked.