I zoom way in to get as close to the line as possible, but at that zoom level you're sort of aiming at the center of a few grey pixels to determine where the line really is. Even if it is off, its only off by probably one or two millimeters, which I don't think would be visible as long as it is smooth and even.
Wow, yeah, that's zoomed! So, similar to boats again - make sure the line or surface is "faired" to the eye, and the numbers don't matter anymore.
This looks fun!