When you are new fretless player, you can use your eyes for reference to train your ears. I use the strobe tuner app called InsTuner (no affiliation) seen below and just rest the iPhone or iPad up against the speaker. I use the lite free version but I think there is another paid version for a buck or two. The strobe is very responsive and accurate but most importantly to fretless players, it provides objective feedback as to whether or not you are perfectly in tune on a given note. This will get you accustomed to placing your fingers precisely in relation to the side dots and other visual cues like wood grain, scratches in the neck if any, etc.
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When you look down at the side of your fingerboard, you see just dots and your fingers in relation to those dots but not really the face of the fingerboard unless you are leaning over to see it. So lines are unnecessary. I only need a dot every other position because visually, I know that I just to have place my finger halfway between two dots to hit a proper note like F#, C#, A#, etc.
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