| In that piece, Jaco uses the natural harmonics that are played on the string between where the 3rd and 4th frets are - so there is a harmonic just above the 3rd fret - which probably equates to 3.2 and one just below the 4th fret - 3.8 .
I wouldn't try measuring it exactly - just find the point where they ring out most clearly and remember it!
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“Making the simple complicated is commonplace; making the complicated simple, awesomely simple, that's creativity.”
Charles Mingus | 
