COAL as listed in most reloading manuals is there due to the SAAMI (or CIP in Euro) spec.
It is NOT however, an accurate measurement for adjusting the amount of freebore that the bullet has to jump through before engaging the lands. One needs to measure THE bullet one intends to use at that bullet's ogive, for a given rifle. Setting the ogive at a given (optimum) distance off the rifle's lands generally improves that rifle's consistency, or precision, resulting in smaller groups.
Since all brands and bullet designs have a different ogive length, this may change with bullet weight within a brand, and when changing brands, or when the load is used in a different rifle.
It is just one more step in making consistent loads, to provide more consistent groups for a given rifle. Even different rifles within a brand (say Remington for instance) may have different freebore, even though they are of the same caliber.
This is also the reason for a custom rifle
's chamber to be spec'd with a known freebore (sometimes known as leade). This allows the bullet of choice to be seated far enough out that it's base doesn't protrude into the case, taking up valuable room for powder, and providing the optimum bullet "jump" (through the freebore) for precision (group size) and pressure control.
A bullet comparator setup and a little experimentation with your rifle and handloads will tell you when you have found your combination's "sweet spot" for the distance off the lands.