Every double bass is different. F2 is not going to in exactly the same spot from one bass to the next and this is especially true when you get up to E3, F3, G3, A3 etc. which are way up the fingerboard on the G-string. So it's not at all like a bass guitar where even fretless ones have position markers. You can do things like look for discoloration spots on the fingerboard to find certain note locations but, of course, that also differs from bass to bass.

When you finger the double bass, use the first, second and fourth fingers. The third should only reinforce the fourth. This permits you to hit the notes precisely instead of microtonals which sound totally out of key. However, when you get up above way the neck joins the body, the fourth finger becomes useless and the thumb has to be positioned on the edge of the neck instead of behind it (the palm should never touch the neck). Also, when you are pressing with the fourth finger, the first, second and third should also be pressing. If you're pressing with the 2nd finger then the 1st should also be pressing. Really, the first finger should ALWAYS be in contact with a string and never floating.

The good news is, this works perfectly well with bass guitar. But if you play bass guitar like a guitarist--using each finger individually--you can't adapt that to double bass because the scale of the latter is so wide that you can't hit notes precisely that way.