Just to be clear, the "fret" is the metal bar. So on a right-handed bass, your white arrow is pointed at the 11th fret. You don't measure from any of the dots you have added.
You measure from where the string comes off the nut...
...to the middle of the 12th fret.
I measure along the course of the G string, which would be where I put the orange arrow.
Doubling that measure will give you the exact scale length. I measure the along the G string because it's the one that will be 30 inches on a 30 inch scale. All the other strings will be a little longer which you compensate for by adjusting the bridge saddles.
Nuts and saddles get measured where the string comes off. The fret gets measured in the middle of the metal bar.
You do the measurement from the nut to the 12th fret to test how long the scale length is. For example, if it is exactly 15 inches you have a 30 inch scale. If it is exactly 15.25 inches, then you have a 30.5 inch scale*. You can then measure (a) from the nut to the saddle, or (b) from the 12th fret to the saddle, whichever is easiest. I usually measure from the nut because I find it easier to rest my straight edge against the nut to do the measurement, but either way works.
*this is common on Gibson short scale basses.
Last edited by fender3x; Today at 06:54 AM.