A micro wiki/primer:
You can generate protractors based on three (quoting this from memory so I may be a bit off on the number) different types of geometries using the Conrad Hoffman generator. The differences of these geometries is in the way they minimise the tracking error. For example, Baerwald tries to have uniform error from the outermost to innermost groove. Similarly, Stevenson tries to minimise error in the innermost grooves.
So besides choosing the geometry, you need to choose what you consider the outer and inner null points of a record. There are multiple standards defining this important data. Vinylengine is a good source of such info.
Once these choices are made, the protractor is generated by the program. The differences in the different protractors will be in different overhang and different offset angles.
Now, the need to adjust overhang requires that the headshell hole must be slotted, and not a single hole like in some headshell designs.
A slotted hole also allows one to adjust the offset angle.
Adjust as per the new protractor and hear how IGD sounds.
Also, please note that cartridge adjustment must be in reference to the cantilever, and not the body of the cartridge itself because the cantilever itself could be slightly misaligned from the factory. Also, not all cartridges have nice square shells that could be used as ready reference.
Good lighting, magnifying glass, patience are key ingredients.
PS: using the Keith Howard Arm Geometer you can generate with your own chosen geometry (do look up Keith Howard's excellent article in Stereophile written when he was a contributing editor, available from the Stereophile archives). It's a bit math heavy but it can be considered a seminal paper on the subject.