Hello,
[TL/DR: Use a calculator and calipers if you want the most precise way of setting an angle]
The intrinsic flaw in using the AngleMaster for knives is described at length in posts such as this one or this other one. The design of the WM-200 offsets the measurement by exactly half the total taper angle of the blade. This is a systematic error that cannot be reduced by training, not by increasing measurement precision of wheel diameter, etc.
The KS-123 is based on a design that allows to measure directly the angle between the tangent to the stone at the point where the apex will sit and the axis of the sharpening jig (which coincides reasonably well with the plane of symmetry of the blade when using the KJ). So no such skew here.
Precision is a different matter. It can come from many things, from the size of the needle on the KS-123 to whether or not you put a light in just the right place when you use the AngleMaster, but also what tolerances cumulate in the construction of the devices, how reproducible the manufacturing process is, etc. etc. To have a substantiated answer to "how precise is the KS-123 ?", you'd need to get a statistically significant sample of jigs, and not only a goniometer but also a consistent way of setting up the tool before you take your measurement. Not sure it's going to happen.
My gut feeling is that the KS-123 has a precision of around 1dps when setting an angle, and probably a fraction of that when reproducing an angle that has been ground using the KS-123 in the first place if your setup process is consistent. In my opinion, the biggest source of error in using the KS-123 comes from the hysteresis/backlash due to the friction between the black plastic part and the metal body and the flexibility oof the plastic needle. It can be reduced with a tad of silicon grease, and consistency can be improved by always approaching the target angle from below by raising the support bar.
Using a calculator rids you of many of these imprecisions. Calipers can be adjusted precisely with very little hysteresis, and the values you use are large enough that a little error will only have a small impact on the result. Much smaller than if you sum the width of the needle with the typical hysteresis of the KS-123.
[TL/DR: Use a calculator and calipers if you want the most precise way of setting an angle]
The intrinsic flaw in using the AngleMaster for knives is described at length in posts such as this one or this other one. The design of the WM-200 offsets the measurement by exactly half the total taper angle of the blade. This is a systematic error that cannot be reduced by training, not by increasing measurement precision of wheel diameter, etc.
The KS-123 is based on a design that allows to measure directly the angle between the tangent to the stone at the point where the apex will sit and the axis of the sharpening jig (which coincides reasonably well with the plane of symmetry of the blade when using the KJ). So no such skew here.
Precision is a different matter. It can come from many things, from the size of the needle on the KS-123 to whether or not you put a light in just the right place when you use the AngleMaster, but also what tolerances cumulate in the construction of the devices, how reproducible the manufacturing process is, etc. etc. To have a substantiated answer to "how precise is the KS-123 ?", you'd need to get a statistically significant sample of jigs, and not only a goniometer but also a consistent way of setting up the tool before you take your measurement. Not sure it's going to happen.
My gut feeling is that the KS-123 has a precision of around 1dps when setting an angle, and probably a fraction of that when reproducing an angle that has been ground using the KS-123 in the first place if your setup process is consistent. In my opinion, the biggest source of error in using the KS-123 comes from the hysteresis/backlash due to the friction between the black plastic part and the metal body and the flexibility oof the plastic needle. It can be reduced with a tad of silicon grease, and consistency can be improved by always approaching the target angle from below by raising the support bar.
Using a calculator rids you of many of these imprecisions. Calipers can be adjusted precisely with very little hysteresis, and the values you use are large enough that a little error will only have a small impact on the result. Much smaller than if you sum the width of the needle with the typical hysteresis of the KS-123.