According to the section on Grain Size at page 42 of Vol. 8 of ASM Metals Handbook 8th Edition, there is a direct correspondence between ASTM E112, intercept method, and the dimension L3 in mm
A direct correspondence with the comparison method is not reported.
You can find this information in chart form in the original journal article published in Metal Progress in April 1938 by R. W. Snyder and H. F. Graff entitled Study of Grain Size in Hardened High Speed . The chart is on page 380.