Following my string change the other day, I realised that the intonation on the bass has drifted off a bit. It is affected by string gauge and design, I might have knocked the saddles during the changeover and, to be honest, it is quite a while since I last checked it.
The clue is that the 12th fret harmonic (an octave above the open string and ringing out when you touch the string exactly at the halfway point) doesn’t match the fretted note at the same position. Or, in the 21st century, one can just use a tuner with a reasonably precise display.
The fix is to move the bridge saddle closer to the bridge if the fretted note is sharp and away from the bridge if it is flat. That moves the half-way point of the string, which should correspond with where it makes contact with the fret when pressed down. You do this after adjusting neck relief and string height, both of which affect how much the string is deflected when being fretted.
It is a fairly quick job with a good set of ears or the aforementioned tuner, although you generally need to take a sounding, slacken the string, make the adjustment and retune before taking another sounding. I’m now much closer than I was before so, even more so than before, can take the blame every time I sound a note that is a little off.