Hey Michael,
the fishing line method is for checking the machine for twist, not for rectangularity. That are two different things that have to be checked independently. For correct alignment when doing tiling or two-sided milling, it’s the rectangularity (“squaring”) of the machine that matters.
One way to check this is the following:
Regarding two-sided projects, a single L bracket is not able to ensure the correct position of the workpiece. That’s why when doing tiling or two-sided milling, you use at least two dowels which should have the greatest possible distance from each other. You don’t forcibly have to drill dowel holes into your wasteboard, you can use a helper board that holds the dowels for a project.