[quote name='Clarky' post='252140' date='Jul 31 2008, 05:16 PM']Hi I have read the threads on intonation today (wish I had read 'em before!) but have seen some recommend using a tuner at the 12th fret for matching the note and harmonic and others say the 19th.
(...)So which fret should it be?[/quote]
Both should work, and give the same result; you could use one and then check it against the other. 24th fret would work too, if your bass has that many.
If they don't yield the same result, it may be because of issues with action, string stiffness, and/or the nut being in a slightly "off" position (which is what the "Buzz Feinten system" is all about); neither of them are things you really can do much about, and oughtn't be noticeable enough to really bother you anyway.
If it does anyway, and you don't use open notes much, you might consider putting a capo on the first fret and intonate at the 13th, 20th and/or 25th fret instead; that will concentrate all the bad intonation on the open notes. OTOH, IMO this fall into the category of "overzealous nitpickery".
EDIT: Someone in the pinned Badass thread on top said that the 19th fret position is preferrable, because, being of a higher pitch, the notes are easier for the tuner to pick out correctly.
Makes sense to me.
OTOH, a natural (overtone/harmonic) 12th (octave + fifth) should be 1902 cents (100ths of a semitone) while the equal-temperament 12th is 1900 cents, or, one fiftieth of a semitone flatter. Of course, that is not a difference that is perceptible by humans (I think the treshold lies somewhere around 20 cents, generally), but, yeah. 24th fret wouldn't have that problem, and is even higher in pitch, for the benefit of the tuner. (Overzealous nitpickery, sure. Or you can view it as amusing trivia.)