You guys aren't going to like my answer.....
A long time ago, I did my own experiments to answer this question: to find out how accurately frets really need to be positioned, to get accurate intonation of every note on an electric bass. The answer I came up with is +/-0.005". I found, by my own experiments, that if you move a fret 0.005", that's enough to see the difference in the frequency reading on a good strobe tuner. 0.005" is 0.127 mm, a little more than a tenth of a millimeter. You can do the experiments yourself, if you don't believe me.
To build a bass that plays all the notes on their correct frequencies, that's the level of accuracy that you need in the positioning of the frets. You need to calculate your chart of numbers to thousandth of an inch (0.001") or tenths of a millimeter (0.1 mm), and then use measuring tools that are accurate to =/- 0.005" or +/- 0.1 mm.
It's not easy to hand-measure to that accuracy over that distance. If you get a good quality machinist's rule, 24" long, the finest graduations you can get are Hundredths of an inch, 0.01". With a magnifier and a fine scribe, you can guess at positions between two of those lines. Metric machinist's rules go as fine as half a mm, 0.5 mm. You have to guess between those lines.
With a good machinist's rule, and using good practice, you can get acceptable accuracy. But machinist's rules aren't cheap. A good quality 24" rule starts at about $60. A really nice Starrett is $388.
If you use one of the
Stew-Mac or other slotting saw jigs with the notched scale rules to slot your fingerboards, they are actually pretty accurate. The notches in the scales are located to about +/- 0.002" accuracy. If the fit on your pin is good and the blade guides are set up snug, you'll be within +/- 0.005" overall.
When I make up notched or scribed scale rules for fret slotting machines, I machine them to an accuracy of +/- 0.002". My own fret slotting saw, with some good technique, will get +/- 0.002".
Out in industry, most instrument manufacturers are surprisingly accurate in fret slot positioning. In the last few decades, anyway. Not so much in the '60's. Fender uses gang saws, which are stacks of saw blades with machined spacers between them. If you measured a modern Fender, it will probably be in the 0.002" range. Most manufacturers, even the imports, use hard tooling like that to cut the fret slots.
Yeah, yeah, I know, I know...Some of you are yelling "Aw, come on, is that kind of accuracy reeeealy necessary on a bass???" In my opinion, yes it is. If the bass is being taken into a studio for recording, poor quality intonation can be measured and recorded and heard. That'll get you some crooked eyebrows from the Engineer and Producer. They may not say anything, but they'll notice.
Accurate fret position is an important part of building a high quality bass. This is one place where you shouldn't say "it's good enough..."
