Ah, so you're worried about the wobble in the horizontal projection. The thing is, because of the way the laser leaves the slit, it's going to be quite smoothed out.

If you just swoop left to right with the pipe and it stays within the meniscus, you're laughing. A 4m projection distance and a 2mm meniscus means that you'd be within 0.05mm (back of the envelope..), which is on par with a ~£100 precision straight edge. To know if it's better than that, just increase the distance further and further - if you can extend it to 10m you're down to <0.03mm for example.

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