Pith wood is best measured as the first 15 years of growth rings, and not as inches. The real concern is the juvenile wood cells, which are usually formed in the first 15 years or so of the tree’s growth.
Today, most logs are 3 or 4rth growth so the growth rings are usually 1/4” wide, or thereabouts, so a 3x3 or 4x4 from the pith usually represents the first 15 years of growth rings.
However, on old growth logs with very tight growth rings, the first 15 years may all be contained in the center 1-1/2” of the log.
Thus, it’s best to count growth rings and use that as a determination of where to edge versus a standard inch measurement.