I don't play fretless but if I were to tackle a fretboard without any markers I would use a technique I have used for fretted for a long time - I have a small hand so have always used all my fingers, including the little one. I use a lot of shapes within an octave span, that is using my 1st finger for bottom note and my little one for the top note. All fits in well in-between and with the thumb at the back of the neck for solid support then the hand position is firm. As has been said, then using ears for intonation and not looking at fretboard, I would hope to make the transfer to fretless a little less painful! Try playing scales, especially chromatic to get every interval there. Glynn