That's what we're trying to convince you of, bro. Lines don't get you to play in tune. They are a rough approximation so you can judge position, not pitch. You can place your finger visually on the line all day long and still be way out. It just ain't that simple. The size and shape of your fingertip. The arch and roll of your joints. The pressure you play with. The setup of your bass. They key of the tune you're playing. The pitches the rest of the ensemble is actually playing (much different from the theoretical!) There are many factors and none of them give a damn about what your hand LOOKS like. YOU CAN'T SEE PITCH. Again, there is exactly ONE applicable tool to evaluate tuning. Your ear. Yes, even an "always on" tuner will lie to you.
I have two position markers on my fretless, 5 and 12. When I need to make a large shift in wrist position I take a glance. My upright has an imperfection in the wood grain almost right where the 4th fret would be, it can be handy for sure but it sure as F doesn't let me know if I'm playing Ab in tune or not!