When I regularly taught new players, I always worked with them to arch their fingers, and to approach the strings from above. It seems that everyone who picks up a bass or guitar, with no previous experience, grabs at the neck. Fingers are laid across it. I'd work on getting my students to move their left thumb to the centre of the back of the neck, and making a curve with their left palm.
With your left fingers striking down on the strings, you can play much faster and smoother, once you get used to the position. Flute and trumpet players are generally taught exactly the same thing, and for the same reason. The flatter your fingers across the stings, the less fluidly you will be able to play.
Looking at my left hand, the calluses are from well above the centre of my finger pads, to just under my fingernails.
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