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[Airic]

I set the string just off the edge of the trench and about 4-6" off the ground, close to the inside edge of one back tire. If you have greater tractor clearance you could come up a bit higher. I bump the string a bit but not enough to rip in out. Only issue is when a dirt claud falls on it pinning it to the ground.

 

[Freds]

Thank you. I hadn't thought about trying to keep it along one edge either. I guess I figured it would do the best job in the center of the trench where the pipe will lay, but after doing a little trenching I can see that the trench is pretty clean along the edges, too.

 

[Wis Bang]

Guys doing foundations/curbs, etc. set up 'batter boards' outside the trench area by a pre-determined distance [like 2'] from each side of the thickness of the wall, etc running paralell strings.

They set this up and square the corners making exact reference points. Nest time you see masons doing a block wall below grade you'll see these lines set up.

Yours would be the offset of the centerline. These guys use those 'folding' wooden rules & a level to keep things in-line.

Just dump all your spoils to the other side.