I've got 100 foot of poly tubing. I fill it almost completely. I leave about a foot of air in either end. Get yourself a reference point and someone on either end. You cannot get it more level than that.
To square the barn I use the ole 6 foot one way, 8 foot the other (90 degrees), and then 10 foot across. It is perfect. 30 foot, 40 foot, and 50 foot works. If you remember HS algebra, its the old A square plus B square equals C square formula for right triangles.
When I built the house, a bunch of civil engineers came out after the foundation was done. They went nuts asking about a transit etc. I laughed at them, told them I did the old fashion way to keep it accurate since I was building it for myself. So they pulled the corners. I was less than an 1/8" out on the corners. They couldn't believe it. Said they had never seen one that accurate. My answer was, get rid of the transit and do it the old fashion way. Daddy and Grandaddy did it the way I did it. Prolly great grandaddy too.
If you want it perfect, get a poly tube full of water and use it. A line level with a perfect bubble can still render you an inch off on a 40 foot span. The water tube is technically an open ended manometer, if you know what a manometer is.
